Widgets need a background color

[Rafał Cieślak]

Almost all widgets allocate some space that is not actually a part of themselves.

This happens when margin is non-zero, when a Box's spacing is non-zero, empty space in Fixed etc.

As for now, that extra space is ignored and not drawn (looks like it remained fully transparent).

All widgets should be able to colorize that area, either with a semi-transparent color (useful in case of Layered dialogs), or a solid color, so that the background may be drawn if needed.

By default all widgets should have a fully transparent background, in order to avoid overrwiting anything that is 'behind' them.