RFR: 8260528: Clean glass-gtk sizing and positioning code [v26]

[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 pull request now contains 20 commits:

 - Small compilation fix for ubuntu 16.04
 - Merge branch 'openjdk:master' into master
 - Merge pull request #18 from openjdk/master
   
   Merge master
 - Fix bug in content oriented child windows
 - Small compilation fix for ubuntu 16.04
 - Replace the window size & positining code
   
   Default to 320x200 if no size is assigned
   
   Hopefully fix all "extra resize" problems due to frame extents.
   
   Small change to reuse get_net_frame_extents_atom()
   
   Fix more tests (restore 1 behaviour)
   
   More test fixes
   
   Partial progress
   
   Adjust positioning (not 100% yet)
   
   It's looking good. Except for fixed initial extents, but it seems a reasonable fix due to nature o frame extents.
   
   It's probably good now
   
   One more small issue
 - Merge pull request #17 from openjdk/master
   
   Pull from origin
 - Merge pull request #16 from openjdk/master
   
   Update
 - Merge pull request #15 from openjdk/master
   
   Update from jfx
 - Merge pull request #14 from openjdk/master
   
   Merge master
 - ... and 10 more: https://git.openjdk.java.net/jfx/compare/78179be1...f4a13314

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Changes: https://git.openjdk.java.net/jfx/pull/367/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=367&range=25
  Stats: 615 lines in 7 files changed: 150 ins; 311 del; 154 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