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) ;
- 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;

In general it reduces code size and complexity and hands more work to gtk.

Important notice: As I could not test the code for handling web start and web applets because browsers do not support it anymore and java has removed "javaws", I took the liberty to remove the code. Will restore if necessary.

Please, mark it as WIP. 

![image](https://user-images.githubusercontent.com/30704286/71788555-442ba400-3002-11ea-9d8c-e4265909fe6b.png)

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Commits:
 - 039daa4a: Cleaning
 - 9f209f1e: JDK-8236651 Simplify and update glass gtk backend

Changes: https://git.openjdk.java.net/jfx/pull/77/files
 Webrev: https://webrevs.openjdk.java.net/jfx/77/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236651
  Stats: 2497 lines in 11 files changed: 600 ins; 1543 del; 354 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