widget: Queue an allocate on hide

widget: Queue an allocate on hide

This makes sure that hidden widgets always have priv->alloc_needed set
on them.

The constructor sets that flag, so we want to have it back when we
revert to this state.

This fixes GtkWindow skipping a size_allocate() when reshowing a
previously hidden window and thereby not updating its allocation and
clip. And that in turn would lead to draws not happening and us beig
left with a black window.