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.