ClutterGroup calls _destroy, but most of St was just calling _unparent.
This caused problems because the DESTROY signal was not emitted
for child elements after destroying a toplevel. Also, in a GC'd
binding it would cause unpredictable lifetime of children.
Some St widgets simply didn't have _dispose at all; implement it.
Note because of the usage of the background_image in StButton,
we can't cleanly destroy it inside the StWidget.
https://bugzilla.gnome.org/show_bug.cgi?id=597845
Convert the StTable code from StStylable to StThemeNode. The
:row-spacing and :col-spacing GObject properties are converted
into spacing-rows and spacing-columns style properties.
A new interactive test is added for StTable.
https://bugzilla.gnome.org/show_bug.cgi?id=596811
Remove the StTable specific methods to add actors:
st_table_add_actor()
st_table_add_actor_with_properties()
Since they shadow the generic ClutterContainer add_actor() method,
and patch in our add() convenience function as we do for
StBoxLayout.
https://bugzilla.gnome.org/show_bug.cgi?id=596811
