[Rev 55] 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 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 20 additional commits
since the last revision:

 - Merge branch 'master' into jdk_8236651
 - Merge pull request #10 from openjdk/master
   
   Update from master
 - Limit GTK on 3.8
 - Limit GTK on 3.18 (Ubuntu 16.04)
 - Forgot a g_print
 - Fix build with merged linux.gradle
 - Merge branch 'master' into jdk_8236651
 - Merge pull request #9 from openjdk/master
   
   Merge from upstream
 - Fix window position bug
 - Rename flag to javafx.gtk.experimental
 - ... and 10 more: https://git.openjdk.java.net/jfx/compare/9f50e9ca...ad8823de

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/77/files
  - new: https://git.openjdk.java.net/jfx/pull/77/files/2700f6fb..ad8823de

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/77/webrev.55
 - incr: https://webrevs.openjdk.java.net/jfx/77/webrev.54-55

  Stats: 14139 lines in 267 files changed: 8389 ins; 4633 del; 1117 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