[Feature request] `always_on_top` configuration for window items

There doesn't seem to be a way to force a window to always be drawn above all other items in the interface unless the item is **focused**. The only item that I've seen behave like this is tooltip items which are always drawn on top of other items without altering focus. It would be nice to have typical window items behave like this when desired for things like transparent overlays or menus -- allowing focus to be maintained where the user desires so these menus don't hide below other items when they aren't the item of focus.

I suggest a new configuration option for window items; something along the lines of `always_on_top`. If set to `True` it will always be drawn above all other windows that have `always_on_top` set to `False`.