RFR: 8236651: Simplify and update glass gtk backend

[Kevin Rushforth]

On Tue, 9 Jun 2020 23:14:31 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> 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.
>
> I am running a full test using GTK 3 on Ubuntu 20.04 and will publish results. I will later do the same for Oracle
> Linux 7.7.
> One thing to note is that this new GTK pipeline doesn't run on Ubuntu 16.04. I get the following running any program:
> 
> $ java -Djavafx.gtk.experimental=true -Djdk.gtk.verbose=true HelloRectangle
> checking GTK version 3
> trying GTK library libgtk-3.so.0
> using GTK library version 3 set libgtk-3.so.0
> Glass GTK library to load is glassgtk3_exp
> loaded gdk_x11_display_set_window_scale
> java: symbol lookup error: /localhome/kcr/javafx/jfx-tmp/jfx/rt/build/sdk/lib/libglassgtk3_exp.so: undefined symbol:
> gdk_display_get_n_monitors

Here are the results with Ubuntu 20.04:

![Ubuntu-20
04-test-results](https://user-images.githubusercontent.com/34689748/84213198-e0a41780-aa74-11ea-9362-08c621af6746.png)

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