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

[Thiago Milczarek Sayao]

This PR is based on top of #915 and 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()`.

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Commit messages:
 - Merge remote-tracking branch 'origin/8292922_drag_events' into 8292922_drag_events
 - Progress
 - Fixes for Ubuntu 16.04
 - 8292922 - [Linux] No more drag events when new Stage is created in drag handler
 - Merge branch 'master' into clean_glass_gtk
 - Merge branch 'openjdk:master' into master
 - Remove unused method
 - Merge remote-tracking branch 'origin/clean_glass_gtk' into clean_glass_gtk
 - Unity uses Compiz
 - Fix show a window minimized
 - ... and 29 more: https://git.openjdk.org/jfx/compare/30147d2f...ddc4c29a

Changes: https://git.openjdk.org/jfx/pull/977/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=977&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292922
  Stats: 855 lines in 6 files changed: 232 ins; 464 del; 159 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