RFR: 8236651: Simplify and update glass gtk backend [v61]

[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 42 additional commits
since the last revision:

 - Merge remote-tracking branch 'origin/jdk_8236651' into jdk_8236651
 - Fix mouse click event
 - Revert to all events mask
 - Small Adjustments
 - Fix compilation on 18.04
 - Merge branch 'master' into jdk_8236651
 - Limit GTK on 3.8
 - Limit GTK on 3.18 (Ubuntu 16.04)
 - Forgot a g_print
 - Fix build with merged linux.gradle
 - ... and 32 more: https://git.openjdk.java.net/jfx/compare/2b740505...a8e1e18b

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/77/files
  - new: https://git.openjdk.java.net/jfx/pull/77/files/ffe3c36e..a8e1e18b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=77&range=60
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=77&range=59-60

  Stats: 392034 lines in 5774 files changed: 194548 ins; 135448 del; 62038 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