Fix issue #3153930: orphan window left on screen

The problem is that if a window containing children is removed
before the children are, the children may be lost.  This change
(amongst the huge amount of new debugging code) now ensures at
this point that all children windows are removed when the parent
is.

Note that this results in a bunch of error messages now as the
client app tries to continue to do things with that child window.
This is correct, it shouldn't be doing that, and needs to be
fixed to stop it.  But at least it now can't cause the window
manager to leak surfaces.

Change-Id: I7b80dd89ff9de7cb5af1dc759cfa4b31ac29cddc