RFR: 8236651: Simplify and update glass gtk backend

[Pankaj Bansal]

On Fri, 12 Jun 2020 05:56:41 GMT, Pankaj Bansal <pbansal at openjdk.org> wrote:

>> Here is the result on Ubuntu 20.04 with the latest changes:
>> 
>> ![image](https://user-images.githubusercontent.com/30704286/84271590-abdea180-ab02-11ea-9d2d-dbca39755db0.png)
>> 
>> Some tests seems intermittent.
>
> I have run the test on OL82 on updated code. Following are the results. I will rerun this on Ubuntu 18.04 and get back
> with the results <img width="647" alt="OL82"
> src="https://user-images.githubusercontent.com/6153953/84469824-918dfa80-ac9f-11ea-9494-36208798312a.png">

I ran the systemTests on OL8.2 using
gradle -PEXTRA_TEST_ARGS='-Djavafx.gtk.experimental=true' -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:cleanTest
:systemTests:test. Following are the results

<img width="578" alt="OL82"
src="https://user-images.githubusercontent.com/6153953/84500229-bdc26f00-acd1-11ea-8d69-2ac3812db636.png">

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