Detangle arrow actions and arrows

[MattWindsor91]

The metamodel is quite confusing when it gets to ArrowSequenceActions. These were originally literally always arrows in the sequence diagram, but now they're used in allowed and forbidden sets - where they neither correspond to arrows nor actions!

This issue tracks overhauling that corner of the metamodel to:

• bring terminology in line with PSCs, where 'arrows' are a type of 'message', alongside 'internal messages' which are the type of thing a gap can contain;
• decouple arrow actions and arrows.

I'm thinking something like:

• Message (has from, to, and body)
  • InternalMessage
  • ArrowMessage
• MessageTopic
  • EventMessageTopic
  • OperationMessageTopic

We might not even need the internal/arrow distinction just yet. I'm imagining the difference will be in the kind of arguments the message can take: arrow messages might be able to bind. Even so, this might be better put as a wellformedness condition rather than a metamodel distinction.

[MattWindsor91]

Fixed by 4e7d5e4 (provisionally at least - the exact layout might change).