RFR: 8260528: Clean glass-gtk sizing and positioning code [v12]

[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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Replace the window size & positining code

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/367/files
  - new: https://git.openjdk.java.net/jfx/pull/367/files/07219e70..6766502e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=11
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=10-11

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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