StTooltip: fix various warnings and use Clutter API correctly

StTooltip: fix various warnings and use Clutter API correctly

Use ClutterContainer functions for adding the tooltip instead of
calling clutter_actor_set_parent behind the stage's back, and do
it inside st_widget_show_tooltip (which is a normal method) instead
of overriding st_tooltip_show, which is a vfunc and it is called
internally by Clutter, therefore it is limited in what it can safely
do.
Also, instead of positioning the tooltip with clutter_actor_set_position,
modify the anchor point when the associated widget moves, so that
only a redraw is queued.

https://bugzilla.gnome.org/show_bug.cgi?id=635100