Overlays should attach to their window (and not the screen) #11
Description
Related to #8
Overlays should have a hard association to the window they are overlaying, for opening in the right spot, being the correct size if we aren't full screen, potentially moving if the window moves, and handling "on top" behavior if neither the overlay nor the target window has focus.
Some of this may be achievable with simple parenting, but this needs investigation with sdl and with how it will work with transparency. Parenting may also give us a way to handle #10