RFR: 8236651: Simplify and update glass gtk backend

[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:
![image](https://user-images.githubusercontent.com/30704286/102702414-1b1b1800-4241-11eb-90bf-8ab737ce2e04.png)

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Commit messages:
 - 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 pull request #13 from openjdk/master
 - Merge pull request #12 from openjdk/master
 - Merge pull request #11 from openjdk/master
 - ... and 7 more: https://git.openjdk.java.net/jfx/compare/f2928d95...bdfd0deb

Changes: https://git.openjdk.java.net/jfx/pull/367/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236651
  Stats: 613 lines in 5 files changed: 136 ins; 311 del; 166 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