background style - not ready to merge

Hi all,

This is my next installment of addressing #711.

This branch works as expected and is almost ready to go in . . . except for a broken spec. (Comments in-line) Has to do with resource disposal . . . perhaps @jasonrclark can help? :)
Thanks for the feedback @PragTob! I know you're super busy with your Thesis. Good luck!

I'm not super familiar with the SWT painter idea, but from what I understand now,  `Background` is creating a `RectPainter` and that the `RectPainter` is looking for [`obj.corners`](https://github.com/shoes/shoes4/blob/master/lib/shoes/swt/rect_painter.rb#L9-L10). So we need to either re-write `RectPainter` or make sure that `SWT::Background` has a corners method.