Extend button widget to support optional "mask", instead of triggering hover, press, etc on entering/clicking the widget boundaries.

[scratch85]

The buttons that I use for the UI are mostly round (yes, the image is a rectangle, but there are transparent parts). One observation I made is that also button widget is a rectangle. Entering the rectangle makes it change from idle to hover, even the button image is not really hovered. It would be nice if one could define an optional mask (circle, ellipse, triangle ...), that actually triggers the hover or press, instead of the pure bounds of the widget.