RFR: 8273379 - GTK3 stops sending key events during drag and drop [v27]

[Thiago Milczarek Sayao]

> This PR fixes 8273379.
> 
> I reverted back to use GDK (from [8225571](https://bugs.openjdk.org/browse/JDK-8225571)) to handle the events. 
> 
> It may also fix [8280383](https://bugs.openjdk.org/browse/JDK-8280383).
> 
> There's also some cleaup.
> 
> To do general testing (two tests were added):
> `java @build/run.args -jar apps/toys/DragDrop/dist/DragDrop.jar`
> 
> Information for reviewing:
> * Previously an offscreen window where used to pass events. Now it gets the window were Drag initially started  (`WindowContextBase::sm_mouse_drag_window`);
> * There's a new `DragSourceContext` instead of global variables;
> * DragView were simplified;
> * It handles `GDK_GRAB_BROKEN` events (I still need to figure it out a test case for this - It should happen when another window grabs the device during the drag);
> * There's a special case for `GDK_BUTTON_RELEASE` because `WindowContext` will notify java about the button release and set `DnDGesture` to null before the end of the DND.
> * `gdk_drag_find_window_for_screen` -> pass the DragView window to be ignored (as it would "steal" destination motion events);
> * The Scenario were the drag source window closes during the drag is now covered;
> * It does not rely on `gdk_threads_add_idle` because it may be inconsistent.

Thiago Milczarek Sayao has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 65 commits:

 - Better end drag
 - Merge branch 'master' into 8273379-dnd-keys
   
   # Conflicts:
   #	modules/javafx.graphics/src/main/native-glass/gtk/glass_dnd.cpp
 - Merge branch 'openjdk:master' into master
 - Do not rely on gdk_threads_add_idle_full to end drag as it might lead to inconsistent behaviour.
 - Revert unnecessary changes
 - Add test scenario
 - Merge branch 'master' into 8273379-dnd-keys
 - Merge branch 'openjdk:master' into master
 - Fix DragView below cursor
 - Minor adjustments
 - ... and 55 more: https://git.openjdk.org/jfx/compare/294e82e6...127677c1

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Changes: https://git.openjdk.org/jfx/pull/986/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=986&range=26
  Stats: 959 lines in 9 files changed: 621 ins; 167 del; 171 mod
  Patch: https://git.openjdk.org/jfx/pull/986.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/986/head:pull/986

PR: https://git.openjdk.org/jfx/pull/986