RFR: 8236651: Simplify and update glass gtk backend

[Thiago Milczarek Sayao]

On Tue, 3 Mar 2020 11:10:07 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> This is going to need further discussion on the mailing list as indicated above, so it is still premature to review it
>> (i.e., it should still be considered effectively a "WIP" until that discussion happens). Additionally, this is a
>> significant and risky change, so I'd like additional eyes on it when we do get to the point of reviewing it.
>
> 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

![image](https://user-images.githubusercontent.com/30704286/77431021-e023ab00-6dba-11ea-9db0-72358a90501b.png)

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