FormEditor: Make it possible to switch from source to form

before designer is fully initialized. The action actually doesn't depend
on designer being initialized, so it can be registered directly at start
up.

Task-number: QTCREATORBUG-11244
Change-Id: I275517befa4dc94714b94a6cd665eb0361a3c45b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>