Add StContainer, which implements the ClutterContainer interface based
on the container methods in st-private and make the existing containers
subclass it.
https://bugzilla.gnome.org/show_bug.cgi?id=613907
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
