UMLR — Need to constrain where triggers can be put in state machines

There does not seem to be any constraint that prevents a transition that does not emanate from an actual State from having a trigger. This means, for instance, that a transition originating on an entry or exit pseudostate, or a history pseudostate, could have a trigger, which is semantically invalid.

A constraint should be added to ensure that only transitions originating from an actual State (except for the Final state) can have a trigger.