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

Updated April 2nd.

![image](https://user-images.githubusercontent.com/30704286/78299385-28a23d80-750c-11ea-9edd-ac264f16c194.png)

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

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