Adds a NoTransition screenmanager transition

This pr proposes a NoTransition transition for the screenmanager that does nothing special and just instantly switches to the new screen.

It's prompted by a conversation with a confused user on irc, and it seems it currently clear isn't how to make screenmanager not use a visible transition. The NoTransition is just a trivial subclass of TransitionBase (with transition duration 0), but it would solve that confusion.