RFR: 8236651: Simplify and update glass gtk backend

Wed Jan 13 13:00:07 UTC 2021

On Sat, 6 Jun 2020 13:03:36 GMT, Pankaj Bansal <pbansal at openjdk.org> wrote:

>> @kevinrushforth Ok, will do as the instructions. Thanks.
>
> I ran the systemTests on Ubuntu 18.04 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="969" alt="Results_18 04" src="https://user-images.githubusercontent.com/6153953/83944842-359d1f00-a824-11ea-964b-f7d9664af9f3.png">

In case it is useful to those reviewing the code, I have created a [WIP pull request](https://github.com/kevinrushforth/jfx/pull/1) in my repo solely for the purpose of looking at the diffs between the existing gtk native code and the experimental GTK native code from *this* pull request.

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