RFR: 8236651: Simplify and update glass gtk backend

[Kevin Rushforth]

On Mon, 6 Jan 2020 00:28:14 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

> ### Summary
> * Simplify and update the Gtk glass backend, making Linux a first-class OpenJFX platform.
> 
> ### Goals
> * Make Linux a first-class OpenJFX platform (see Motivation);
> * Simplify the code and reduce it's size;
> * Update to gtk3 (it was originally a port from gtk2);
> * Remove unused code (such as applets and web start);
> * Prepare the ground for a possible future Wayland support.
> ### Testing
> ./gradlew -PEXTRA_TEST_ARGS='-Djavafx.gtk.experimental=true' -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:test

> Please, mark it as WIP.

You can do this by editing the title and adding `WIP: ` as a prefix.

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