Uniter API now has WatchActions

This commit adds state/api and state/apiserver endpoints for
WatchActions.
I'm not entirely clear on how to mock up the API call or what pieces need to be assembled for that, versus what pieces are already there.  Perhaps we can chat about that tomorrow.

Part of what's confusing me here is that the WatchActions call is a method on Unit, and I don't really see how to put together a Unit that won't have a valid WatchActions call, yet will be a valid Unit.