Remove Evoral::MIDIEvent

Remove Evoral::MIDIEvent

It is slightly questionable whether type specific methods like
velocity() belong on Event at all, these may be better off as free
functions.  However the code currently uses them as methods in many
places, and it seems like a step in the right direction, since, for
example, we might some day have events that have a velocity but aren't
stored as MIDI messages (e.g. if Ardour uses an internal musical model
that is more expressive).

In any case, the former inheritance and plethora of sloppy casts is
definitely not the right thing.