[Rev 53] RFR: 8236651: Simplify and update glass gtk backend

[Thiago Milczarek Sayao]

> ### Summary
> * Simplify and update the Gtk glass backend, making Linux a first-class OpenJFX platform.
> 
> ### Goals
> * Make Linux a first-class OpenJFX platform (see Motivation);
> * Simplify the code and reduce it's size;
> * Update to gtk3 (it was originally a port from gtk2);
> * Remove unused code (such as applets and web start);
> * Prepare the ground for a possible future Wayland support.
> ### Testing
> ./gradlew -PEXTRA_TEST_ARGS='-Djavafx.gtk.experimental=true' -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:test

Thiago Milczarek Sayao has updated the pull request incrementally with one additional commit since the last revision:

  Limit GTK on 3.18 (Ubuntu 16.04)

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/77/files
  - new: https://git.openjdk.java.net/jfx/pull/77/files/e947aa66..9c83ac12

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/77/webrev.53
 - incr: https://webrevs.openjdk.java.net/jfx/77/webrev.52-53

  Stats: 80 lines in 4 files changed: 1 ins; 76 del; 3 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