RFR: 8236651: Simplify and update glass gtk backend

[Tor (torbuntu)]

On Fri, 3 Jul 2020 13:49:54 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> @pankaj-bansal Sorry for commiting again. Now the Tab Pane test works properly.
>
>> If you want these commits to be preserved when you integrate, you will need to make a 'merge-style' pull request.
> 
> Please ignore this information from Skara. I don't know what caused it, but I've pinged the Skara folks about it.

Would it be possible and related to add the gtk touch events to this PR as part of the Glass GTK3 update? Or would
follow up update PR adding that be more appropriate?

Currently linux desktop does not have touch support because of the missing gesture/touch events from gtk3.

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