Actions watcher redux

Make sure Actions queued before a watcher starts are picked up when the watcher does start.

Also slight refactoring to prepare for Actions being queued on Services as well as Units
The ActionReceiver interface gives us an abstract way to think about Entities/etc. that can have Actions queued for them.  Right now state.Unit is the only implementer of this interface, but soon (after refactoring the names.ActionTag class) this will be implemented by state.Service too