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 April 2nd.
> 
> ![image](https://user-images.githubusercontent.com/30704286/78299385-28a23d80-750c-11ea-9edd-ac264f16c194.png)

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

Updated April 2nd.

![image](https://user-images.githubusercontent.com/30704286/78316727-c14db300-7536-11ea-86e9-4d5c56e4d92c.png)

Note: DatePickerTest works when run alone.

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