Don't lose the focus when refreshing a thread

[lenormf]

This commit allows keeping the currently focused message selected
across thread refreshes.

Since this operation can take a long time in large threads, a
search_threads_rebuild_limit option is introduced. When defined,
it sets the maximum amount of messages that will be iterated upon
when trying to find the previously focused message.

[pazz]

This is obviously a hack but it patches over some intrinsic urwid/notmuch problems that lots of users seem to care about, so I'm willing ot add it.

Please fix the avoidable codeclimate issues where possible.
I've triggered travis to re-run the docs generation. Something fails there. This needs to be fixed before merging.

[lenormf]

I fixed the codeclimate issue I could, it looks like some Travis builds fail because of documentation problems? Have I edited the wrong files?

[pazz]

Thanks. Did you rebase to master already? I believe these travis-docs issues are fixed on master..

[lenormf]

I rebased on b11aa98, still the same build errors :/

[pazz]

Ah right: you've changed some config options but did not re-generate the docs locally (which will generate some tables that need to be checked in).

make -C docs cleanall html

I saw that you did this for your original R but perhaps not after the rebase..
Also, could you try and avoid the lines-too-long error if possible?

[pazz]

This looks good to me now, but with out having tested this.
I'd appreciate some positive user test before I merge this...

[pazz]

I've tested this myself just now and it looks quite good.
One problem, which we need not necessarily fix here, is that there is no perceivable user feedback upon a refresh in search mode.

It should be nice do include a similar patch for thread mode as well, where I personally use the refresh command much more often to check on new mails in a thread.

It seems to have exposed a (unrelated?) bug in the email module for me:
search for * and scroll down a few pages, then hit @ for refresh.
The logs show this trace (and I get a corresponding popup) occasionally:

 File "/home/pazz/projects/alot/alot/ui.py", line 723, in apply_command
    cmd.apply(self)
  File "/home/pazz/projects/alot/alot/commands/globals.py", line 172, in apply
    ui.update()
  File "/home/pazz/projects/alot/alot/ui.py", line 668, in update
    self.mainloop.draw_screen()
  File "/usr/lib/python3/dist-packages/urwid/main_loop.py", line 586, in draw_screen
    canvas = self._topmost_widget.render(self.screen_size, focus=True)
  File "/usr/lib/python3/dist-packages/urwid/widget.py", line 144, in cached_render
    canv = fn(self, size, focus=focus)
  File "/usr/lib/python3/dist-packages/urwid/decoration.py", line 226, in render
    canv = self._original_widget.render(size, focus=focus)
  File "/usr/lib/python3/dist-packages/urwid/widget.py", line 144, in cached_render
    canv = fn(self, size, focus=focus)
  File "/usr/lib/python3/dist-packages/urwid/container.py", line 1085, in render
    body = self.body.render((maxcol, maxrow-ftrim-htrim),
  File "/home/pazz/projects/alot/alot/buffers/buffer.py", line 19, in render
    return self.body.render(size, focus)
  File "/usr/lib/python3/dist-packages/urwid/widget.py", line 144, in cached_render
    canv = fn(self, size, focus=focus)
  File "/usr/lib/python3/dist-packages/urwid/listbox.py", line 470, in render
    middle, top, bottom = self.calculate_visible(
  File "/usr/lib/python3/dist-packages/urwid/listbox.py", line 416, in calculate_visible
    next, pos = self._body.get_next( pos )
  File "/home/pazz/projects/alot/alot/walker.py", line 46, in get_next
    return self._get_at_pos(start_from + self.direction)
  File "/home/pazz/projects/alot/alot/walker.py", line 72, in _get_at_pos
    widget = self._get_next_item()
  File "/home/pazz/projects/alot/alot/walker.py", line 85, in _get_next_item
    next_widget = self.containerclass(next_obj, **self.kwargs)
  File "/home/pazz/projects/alot/alot/widgets/search.py", line 26, in __init__
    self.rebuild()
  File "/home/pazz/projects/alot/alot/widgets/search.py", line 60, in rebuild
    width, part = build_text_part(partname, self.thread,
  File "/home/pazz/projects/alot/alot/widgets/search.py", line 145, in build_text_part
    content = prepare_string(name, thread, maxw)
  File "/home/pazz/projects/alot/alot/widgets/search.py", line 213, in prepare_string
    s = content(thread)
  File "/home/pazz/projects/alot/alot/widgets/search.py", line 178, in prepare_authors_string
    return thread.get_authors_string() or '(None)'
  File "/home/pazz/projects/alot/alot/db/thread.py", line 195, in get_authors_string
    for aname, aaddress in self.get_authors():
  File "/home/pazz/projects/alot/alot/db/thread.py", line 158, in get_authors
    msgs = sorted(self.get_messages().keys(),
  File "/home/pazz/projects/alot/alot/db/thread.py", line 247, in get_messages
    self._toplevel_messages.append(accumulate(self._messages, m))
  File "/home/pazz/projects/alot/alot/db/thread.py", line 237, in accumulate
    M = Message(self._dbman, msg, thread=self)
  File "/home/pazz/projects/alot/alot/db/message.py", line 71, in __init__
    self.mime_part = get_body_part(self.get_email())
  File "/home/pazz/projects/alot/alot/db/utils.py", line 491, in get_body_part
    body_part = mail.get_body(preferencelist)
  File "/usr/lib/python3.8/email/message.py", line 1018, in get_body
    for prio, part in self._find_body(self, preferencelist):
  File "/usr/lib/python3.8/email/message.py", line 989, in _find_body
    yield from self._find_body(subpart, preferencelist)
  File "/usr/lib/python3.8/email/message.py", line 989, in _find_body
    yield from self._find_body(subpart, preferencelist)
  File "/usr/lib/python3.8/email/message.py", line 978, in _find_body
    if part.is_attachment():
AttributeError: 'str' object has no attribute 'is_attachment'

[pazz]

It seems my last issue is unrelated. I have created a new issue for his (#1506).