Deprecate JavaFX GTK 2 library for removal

With the announcement of the GTK 4 release in December 2020, the GTK 2 toolkit has reached its end of life.

https://blog.gtk.org/2020/12/16/gtk-4-0/

I propose to deprecate the JavaFX glass GTK 2 backend for removal. Since there isn't an associated API, the mechanism for doing this will be to print a "deprecated for removal" warning message when the glassgtk2 library it is loaded. A message should be printed regardless of why the GTK 2 library was loaded, which can happen for one of the following reasons:

1. The application or end user has requested GTK 2 using the "jdk.gtk.version" system property.
2. Another native library has already loaded the GTK 2 library (an application cannot load both the GTK 2 and GTK 3 libraries in the same process)
3. As a fallback if the gtk3 libraries are not available on the system.

We might consider slightly different messages in each case, but the important thing is to clearly note that GTK 2 support will be removed in future versions.

I propose to deprecate this for removal in JavaFX 18. We can then remove it in JavaFX 19 or, more likely, JavaFX 20.