RFR: 8292922 - [Linux] No more drag events when new Stage is created in drag handler [v4]

[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:

  Oh this was it

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Changes:
  - all: https://git.openjdk.org/jfx/pull/977/files
  - new: https://git.openjdk.org/jfx/pull/977/files/94645978..63ee8815

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=977&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=977&range=02-03

  Stats: 27 lines in 2 files changed: 0 ins; 1 del; 26 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