[Rev 31] 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)

Thiago Milczarek Sayao has updated the pull request with a new target base due to a merge or a rebase. The incremental
webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits
since the last revision:

 - Merge branch 'master' into jdk_8236651
 - Merge pull request #8 from openjdk/master
   
   Merge upstream
 - fix compilation on ubuntu 16.04
 - Merge remote-tracking branch 'origin/jdk_8236651' into jdk_8236651
 - Merge branch 'master' into jdk_8236651
 - Merge pull request #7 from openjdk/master
   
   merge from jfx

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/77/files
  - new: https://git.openjdk.java.net/jfx/pull/77/files/086143ac..f8fe264c

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/77/webrev.31
 - incr: https://webrevs.openjdk.java.net/jfx/77/webrev.30-31

  Stats: 1810 lines in 39 files changed: 1206 ins; 249 del; 355 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