st-widget: Implement a proper allocate

Since an StWidget now has children, it needs to allocate those children
properly. Defer to the currently installed layout manager, like Clutter
does.

Now that we have something that allocates children in St, to prevent
double allocations, we use clutter_actor_set_allocation rather than
chaining up to StWidget::allocate.

https://bugzilla.gnome.org/show_bug.cgi?id=670034