This resolves a PBD vs ARDOUR namespace error for some compilers:
```
error: reference to 'microseconds_t' is ambiguous
libs/pbd/pbd/microseconds.h:29:19: error: candidates are: typedef uint64_t PBD::microseconds_t
libs/ardour/ardour/types.h:81:29: error: typedef PBD::microseconds_t ARDOUR::microseconds_t
```
This also removes enums introduced to describe well-known parameters for Mixbus. Lookup now involves string
parsing every time, but this is not likely to be a notable cost.
The Editor continues to notify them, but via a direct call to ControlProtocolManager, not a signal.
The CP Manager calls the ControlProtocol static method to set up static data structures holding
selection info for all surfaces and then notifies each surface/protocol that selection has changed.