RFR: 8236651: Simplify and update glass gtk backend

[Thiago Milczarek Sayao]

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

> This proposed change does the following:
> 
> - Ports DND target to use GTK reducing code size and adding extra text / image formats (such as .gif);
> - Use gtk signals instead of gdk events (also to reduce code size);
> - Simplifies geometry (sizing/positioning) with a more straightforward code (less special cases) ;
> - Replaces (pointer and focus) grabbing with a gtk approach;
> - Reworked frame extents (the wm extension to get decoration sizes) to reduce size and complexity;
> - Simplified cursor changing;
> - Reduced the use of gtk/gdk deprecated functions;
> 
> 
> In general it reduces code size and complexity and hands more work to gtk.
> 
> Important notice: As I could not test the code for handling web start and web applets because browsers do not support it anymore and java has removed "javaws", I took the liberty to remove the code. Will restore if necessary.
> 
> Please, mark it as **WIP**. 
> 
> ![image](https://user-images.githubusercontent.com/30704286/71791073-58779d00-3012-11ea-89e5-07588f7a41cc.png)

There are still some sizing issues that I am aware of.

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