RFR: 8273379 - GTK3 stops sending key events during drag and drop [v3]

[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.
> 
> The most changed part is that I had to move `process_events` to `glass_evloop`  because it's reused in glass_dnd.
> 
> To do general testing:
> `java @build/run.args -jar apps/toys/DragDrop/dist/DragDrop.jar`

Thiago Milczarek Sayao has updated the pull request incrementally with one additional commit since the last revision:

  Revert "Fix image drag"
  
  This reverts commit 635cb89a196c928a8b807a3ff90dca99f915e5ac.

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/986/files
  - new: https://git.openjdk.org/jfx/pull/986/files/635cb89a..6c2d5139

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=986&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=986&range=01-02

  Stats: 23 lines in 2 files changed: 17 ins; 0 del; 6 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