fix up some const-ness issues starting from Evoral::Event::set(), and intersect with...

fix up some const-ness issues starting from Evoral::Event::set(), and intersect with removing Mackie..MidiByteArray::bytes() method which made a copy of the data every time we wrote it

git-svn-id: svn://localhost/ardour2/branches/3.0@12124 d708f5d6-7413-0410-9779-e7cbd77b26cf