Less type safety: remove Handler::Msg assoc. type

[dhardy]

When is type safety not a good thing? This PR removes the Handler::Msg type and Response::Msg variant, replacing this with a new variadic message queue.

• Add EventMgr::push_msg, try_pop_msg (+ boxed versions)
• Remove Response::Update and Updatable trait
• Remove Handler::Msg, Response::Msg, VoidMsg, #[derive(VoidMsg)
• Remove #[widget(update = f)], #[widget(map_msg = f)], etc. (#[widget] attr on fields no longer accepts parameters)
• Button, CheckBox etc. closures no longer return Option<M> but push directly via mgr.push_msg(..)
• Remove IndexedList, IndexedColumn, IndexedRow, GenericList
• Remove Response::Select; add SelectMsg
• Remove Response::Scrolled, Pan and Focus variants; new Scroll enum
• Replace dyn WidgetConfig with dyn Widget
• Remove SendEvent; replace with EventMgr::send method (widgets no longer handle event routing)
• Warn in log if a message is unhandled, with value of message (debug only)
• Add Handler::handle_message: called on parent whenever a child emits a message
• Add List::on_message and Grid::on_message; optionally call closure with child index when child emits a message
• Remove Driver::get
• *Data traits: replace update_handles with update_on_handles (pass &WidgetId in and let data register dependencies), update method takes &mut EventMgr and notifies of update instead of returning an UpdateHandle
• Add *Data::handle_message, called by view widget when a child emits a message (replacement for Response::Update and Updatable)

There are also a few fixes / minor behaviour changes:

• ComboBox widget: if menu entries have accelerator keys, these are now only usable when the menu is open (same as other menus)
• ComboBox: fix highlighting of first entry
• ComboBox: constructor no longer takes active parameter; all entries have an associated message (of the same type)
• examples/mandlebrot: enable tab focus of main widget (allows full keyboard control)
• Splitter widget: fix child index to WidgetId assignment
• Fix widget derive: implement all methods
• Slider widget: clamp handle to detents
• Handler::handle renamed to handle_event
• Prettier WidgetHeirarchy output
• Fix weird width/height interaction in RowSetter for columns

Effects and motivation

With over 40 commits and approx 2000 line insertions and 3000 deletions, this is not a small PR, and a significant step in trait revision (the rough topic for v0.11). Effects:

• There is no longer a compile-time error when forgetting to handle a message from a child widget. Instead, there is only a warning in the log. This is a definite regression, albeit not too important I think (some more testing will be required when building UIs).
• Multiple message types may be emitted by the same widget (or by a parent alongside a child's message), and multiple concurrent messages may be placed on the stack. Minor improvement.
• One less type parameter. dyn Widget is now possible without parametrizing the message type. Less type parameters to worry about implies simpler code and simpler error messages.
• Event routing is now done by the library; no more widget-specific SendEvent impls! As a consequence, the Handler trait gained handle_message, handle_unused and scroll methods.
• Data models/view widgets changed significantly with regards to handling user input, with roughly the same capability and possibly slightly better code.

Effects on user code not using data models is limited, mostly to not needing to specify message types or handler annotations (e.g. #[widget(use_msg = start)]).

Closes #11, #257.

Bugs

Tab order broken in examples filter-list and data-list-view when paging. Warnings are logged, e.g.:

[2022-04-26T08:28:01Z WARN  kas_core::event::manager] Widget ListView#052 cannot find path to #0529310
[2022-04-26T08:28:02Z WARN  kas_core::event::manager] Widget ListView#052 cannot find path to #0529220

I will leave this to a future PR since this one is otherwise ready.

MSRV

Bump min Rust version to 1.58, only for the new format syntax (embed identifiers).