I'm not sure if this is really the best way to do event types (should it
just be a completely static enum in evoral, or completely dynamic and
provided by the type map, or a mix like currently?), but previously the
event type was frequently set to either total garbage, or parameter
types, which are a different thing.
This fixes all those cases, and makes Evoral::EventType an enum so the
compiler will warn about implicit conversions from int.
398a318934