dashSpacer: Don't trigger allocations from size negotiations

If an actor's allocation is outdated, clutter_actor_get_allocation_box() will queue a relayout. That's why it's advised to not use the function unless the allocation is known to be valid (namely during paint), but in particular not from within get_preferred_width/height vfuncs.

Using the :allocation property (which may be outdated) would be better, but in this case we can simply delegate the request to the correct actor.

Fixes #1065 (closed)