Efficiently maintaining a "boundsIncludingChildren" property

[davidedc]

...this could allow a faster identification of which (parts of) morphs need to be redrawn to fix the "broken" rectangles.
Note that in case you do this, you probably also want to add a flag to each morph to indicate whether it has a FrameMorph (or any of its subclasses) in any of its parents (or itself, obviously).