RFR: 8292922 - [Linux] No more drag events when new Stage is created in drag handler [v6]

[Thiago Milczarek Sayao]

> This PR  was previously discussed on #905.
> 
> The approach is to grab the keyboard focus so the window that originated the drag will keep it.
> 
> I did some cleanup on grabbing related functions as well.
> 
> `gdk_keyboard_focus()` is deprecated, so is `gdk_device*` functions in favor of `gdk_seat*`. But that's only available in later Gtk versions. I checked and newer Gtk will use `gdk_seat*` inside the deprecated `gdk_keyboard_focus()`.

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

  Grab drag focus on drag

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/977/files
  - new: https://git.openjdk.org/jfx/pull/977/files/c02f1b97..35c45b09

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=977&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=977&range=04-05

  Stats: 17 lines in 1 file changed: 8 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/977.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/977/head:pull/977

PR: https://git.openjdk.org/jfx/pull/977