Allow non-sequential combinedfragments to select a subset of actors

[MattWindsor91]

In discussion with Ana, a useful way to handle the issues associated with a timeout stranding ends of messages on a different lifeline would be to have an opt, alt or xalt (presumably the third) that surrounds the ends that are not inside the timeout.

Contrary to what I initially thought, we shouldn't need to complicate the messages here, as the fragment would technically encompass the entire MessageFragment but only affect some ends of it.

This, in and of itself, should not be a hard metamodel or semantic change. The issues would be in the notations. Textually we'd need a concise way of specifying which actors are affected. Graphically we'd need to handle the possibility of non-consecutive lifelines (either by forbidding them or by having extra notation for them).