drag and drop: support images, text, server-to-client, etc

[totaam]

Follow up from #1493.

• we have the source window and target window, so we should be able to send the data to the specific application window that was pointed to (if it supports it... which we won't know about until it is too late?)

• we should handle more data types and sanitize them: the "selection-data" structure can give us plain text, UTF8, images, etc.
• this same code could be re-used for sanitizing clipboard data (ie: images)
• on the server-side, we may be able to register the "world-window" as a drop target so that we can trigger drag-n-drop events from server to client

[totaam]

Examples of targets with common applications:

• browser URL:

'STRING', 'UTF8_STRING', 'text/plain', 'text/x-moz-url', 'TEXT', '_NETSCAPE_URL'

• browser text data:

'text/x-moz-url', '_NETSCAPE_URL', 'application/x-moz-custom-clipdata', 'text/unicode', 'text/plain;charset=utf-8', 'text/plain', 'text/html'

• browser image:

STRING, UTF8_STRING, XdndDirectSave0, text/plain, text/x-moz-url, TEXT, text/html, chromium/x-renderer-taint, application/octet-stream

Docs:

• Drag-and-Drop Protocol for the X Window System: Dropping on the root window
• GTK : DND Methods
• GTK : Drag And Drop

[totaam]

Links:

• XDND: Drag-and-Drop Protocol for the X Window System
• Gtk.Widget drag_begin API we can't use since we don't have a gtk window for foreign windows (only a gdk window)
• gdk.Window.drag_begin and gdk.DragContext
• pygtk2 tutorial: DND Methods
• gtk.Widget drag-begin

Related to #812.

[totaam]

Updates in #812 comment:27

[totaam]

FWIW, it may be easier to start by supporting file transfers via the clipboard.
Using the default file manager on Linux, it offers the following types when copying a file to the clipboard:

SAVE_TARGETS, TIMESTAMP, TARGETS, x-special/gnome-copied-files, application/vnd.portal.filetransfer, 
application/vnd.portal.files, text/uri-list, UTF8_STRING, COMPOUND_TEXT, TEXT, STRING, text/plain;charset=utf-8

This could work more easily for X11:

• expose the text/uri-list clipboard TARGET
• when the remote application asks for this target:
  • transfer the files to a temporary directory
  • respond with a custom text/uri-list pointing to these files

This can only work if we have enough time to copy the files over before replying to the selection request, so only for small-ish files + small number of files.

The FileTransfer portal is another, more heavyweight solution.

Interestingly, the HTML drag and drop API also uses text/uri-list: HTML Drag and Drop API: Recommended drag types