fix replacewithgapfill
fix replacewithgapfill

something in the order of things was causing part.path to be recomputed
during replaceWithGapfill, before the new gap's parent was set.

This moves the line changing levelName() to the end of
replaceWithGapfill, so that path isn't recomputed until everything's set
up.

fixes #549

numbas/editor,numbas/editor,numbas/editor