RFR: 8260528: Clean glass-gtk sizing and positioning code [v27]

[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 incrementally with one additional commit since the last revision:

  Fix build
  gtk_widget_set_sensitive has no effect

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/367/files
  - new: https://git.openjdk.java.net/jfx/pull/367/files/f4a13314..9ff5a7b6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=26
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=25-26

  Stats: 3 lines in 1 file changed: 1 ins; 1 del; 1 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