In StBin, StBoxLayout, and StTable, if a child has a potential
allocation that is larger than its preferred size, we give it its
preferred size instead. However, the corresponding
get_preferred_height/width methods were not making the same
assumption, which meant that if we had more width than the widget
wanted, we would allocate it its preferred width, but with the height
that corresponded to the larger width.
Fix this by defining new helpers _st_actor_get_preferred_width() and
_st_actor_get_preferred_height() and using them everywhere. Also, make
StBin and StTable use _st_allocate_fill() rather than having
nearly-identical duplicate copies of the code.
https://bugzilla.gnome.org/show_bug.cgi?id=609848
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
