RFR: 8236651: Simplify and update glass gtk backend

[Thiago Milczarek Sayao]

On Tue, 24 Mar 2020 13:33:33 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> I have been testing this for several days on ubuntu 18.04 and it's working good. It pass system tests, runs Ensemble 8
>> and Scenebuilder.
>> Will have to do some tests on 16.04.
>
> Ubuntu 20.04 Test Results
> 
> Updated 31 march 2020.
> 
> ![image](https://user-images.githubusercontent.com/30704286/78094606-5459e200-73ab-11ea-8de9-e6c1c569c318.png)
> 
> Note: ColorPickerTest worked on a "solo" run,

Test on 16.04 (without Webkit - it does not build on 16.04 anymore)

![image](https://user-images.githubusercontent.com/30704286/78095942-b9fb9d80-73ae-11ea-9dfa-108daa3479e5.png)

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

PR: https://git.openjdk.java.net/jfx/pull/77