RFR: 8260528: Clean glass-gtk sizing and positioning code

[Thiago Milczarek Sayao]

On Thu, 18 Feb 2021 21:14:44 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> Changed to WIP to reduce sizing events.
>
> Current Status:
> 
> ![image](https://user-images.githubusercontent.com/30704286/108422319-2aa0e800-7215-11eb-8b9e-8f62e7f8d553.png)

Ready to be reviewed.

-------------

PR: https://git.openjdk.java.net/jfx/pull/367