Wrap MusicalTime in a class.

This lets us get a more explicit handle on time conversions, and is the main
step towards using actual beat:tick time and getting away from floating point
precision problems.

libs/evoral/src/Event.cpp

@@ -156,6 +156,7 @@ Event<Timestamp>::set_original_time (Timestamp t)
 #endif // EVORAL_EVENT_ALLOC
 
 template class Event<Evoral::MusicalTime>;
+template class Event<double>;
 template class Event<int64_t>;
 
 } // namespace Evoral