[Rev 18] RFR: 8236651: Simplify and update glass gtk backend

[Thiago Milczarek Sayao]

> This proposed change does the following:
> 
> - Ports DND target to use GTK reducing code size and adding extra text / image formats (such as .gif);
> - Use gtk signals instead of gdk events (also to reduce code size);
> - Simplifies geometry (sizing/positioning) with a more straightforward code (less special cases), making it easier to understand and maintain;
> - Replaces (pointer and focus) grabbing with a gtk approach;
> - Reworked frame extents (the wm extension to get decoration sizes) to reduce size and complexity;
> - Simplified cursor changing (for gtk3);
> - Reduced the use of gtk/gdk deprecated functions;
> - Removes Applet/Web Start code;
> - Fixes https://bugs.openjdk.java.net/browse/JDK-8237491;
> 
> In general it reduces code size and complexity and hands more work to gtk.
> 
> Updated on 2020-01-29:
> ![image](https://user-images.githubusercontent.com/30704286/73354728-2ce47d00-4275-11ea-935c-414fc26163d7.png)

The pull request has been updated with a new target base due to a merge or a rebase.

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

Commits:
 - 14a75ce4: Merge branch 'master' into jdk_8236651
 - e829b8e9: Better fix for initial size
 - 058a0992: Revert "Fix Initial Size"
 - 0c982d60: Fix Initial Size
 - 694641f6: JDK-8236651 Simplify and update glass gtk backend
 - 9397a748: Merge pull request #4 from openjdk/master

Changes: https://git.openjdk.java.net/jfx/pull/77/files
 Webrev: https://webrevs.openjdk.java.net/jfx/77/webrev.18
  Stats: 3360 lines in 12 files changed: 782 ins; 1797 del; 781 mod
  Patch: https://git.openjdk.java.net/jfx/pull/77.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/77/head:pull/77

PR: https://git.openjdk.java.net/jfx/pull/77