st-im-text: Chain up to parent first in dispose()

st-im-text: Chain up to parent first in dispose()

The actor's GtkIMContext is freed in dispose and reset in unrealize - as
ClutterActor's dispose will unrealize the actor if necessary, chaining
up to the parent after clearing the im context will result in warnings
if the actor is still realized, so chain up first.

https://bugzilla.gnome.org/show_bug.cgi?id=686016