Fixed GtkWindow/GtkWidget to properly emit hierarchy changed for embedded toplevels
Now GtkWindow takes some measures when setting toplevelness:
- When a window becomes toplevel after being embedded it saves
the visibility state and reshow's itself so that the window
re-realizes and presents itself again automatically
- When emitting hierarchy-changed, synthetically mark the toplevel
as not anchored, this allows the hierarchy changed propagation to
recurse properly.
GtkWidget also takes care to unset the parent window *after* unparenting
the widget and after emitting the heirarhcy changed that leaves a NULL
toplevel.
That means there are now 2 cycles of "hierarchy-changed" when removing
an embedded toplevel from a parent, first one that makes the new toplevel
a NULL one (since the toplevel flag is not yet restored), the second cycle
makes the removed window toplevel again when setting the parent window
to NULL.