actor: Add internal child flag

ClutterActor checks, when destroying and reparenting, if the parent
actor implements the Container interface, and automatically calls the
remove() method to perform a clean removal.

Actors implementing Container, though, might have internal children;
that is, children that are not added through the Container API. It is
already possible to iterate through them using the Container API to
avoid breaking invariants - but calling clutter_actor_destroy() on
these children (even from the Container implementation, and thus outside
of Clutter's control) will either lead to leaks or to segmentation
faults.

Clutter needs a way to distinguish a clutter_actor_set_parent() done on
an internal child from one done on a "public" child; for this reason, a
push/pop pair of functions should be available to Actor implementations
to mark the section where they wish to add internal children:

  ➔ clutter_actor_push_internal ();
    ...
    clutter_actor_set_parent (child1, parent);
    clutter_actor_set_parent (child2, parent);
    ...
  ➔ clutter_actor_pop_internal ();

The set_parent() call will automatically set the newly added
INTERNAL_CHILD private flag on each child, and both
clutter_actor_destroy() and clutter_actor_unparent() will check for the
flag before deciding whether to call the Container's remove method.

doc/reference/clutter/clutter-sections.txt

@@ -352,6 +352,8 @@ clutter_actor_lower
 clutter_actor_raise_top
 clutter_actor_lower_bottom
 clutter_actor_get_stage
+clutter_actor_push_internal
+clutter_actor_pop_internal
 
 <SUBSECTION>
 clutter_actor_set_depth