RFR: 8236651: Simplify and update glass gtk backend [v2]

[Thiago Milczarek Sayao]

> This is a new approach to rewrite parts of gtk glass backend to be more clean.
> 
> I will provide small "manageable" PR to incrementally make the backend better.
> 
> This PR adresses cleanup of the Size and Positioning code. It makes code more "straightforward" and easier to maintain.
> 
> Current status (Ubuntu 20.04):
> ![image](https://user-images.githubusercontent.com/30704286/102702414-1b1b1800-4241-11eb-90bf-8ab737ce2e04.png)
> 
> (*) Some of the iconify tests are also failing on the main branch. 
> 
> `gradlew -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:test --tests test.robot.javafx.stage.IconifyTest` on a second run produces 4 tests, 2 failures.

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 glass_gtk_new_position_and_size
 - Merge pull request #15 from openjdk/master
   
   Update from jfx
 - Avoid redundant resize notify
 - Fix parent window being resizable (it should not)
 - Minor fix to positioning
 - Small adjustment
 - Fixes
 - Revert files
 - Replace the window size & positining code
 - Merge pull request #14 from openjdk/master
   
   Merge master
 - ... and 10 more: https://git.openjdk.java.net/jfx/compare/d304d2ea...2be1ba7e

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/367/files
  - new: https://git.openjdk.java.net/jfx/pull/367/files/bdfd0deb..2be1ba7e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=00-01

  Stats: 15117 lines in 291 files changed: 1191 ins; 12297 del; 1629 mod
  Patch: https://git.openjdk.java.net/jfx/pull/367.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/367/head:pull/367

PR: https://git.openjdk.java.net/jfx/pull/367