introduce the notion that note additions and property changes can cause the removal...

introduce the notion that note additions and property changes can cause the removal of other notes because of overlaps; merge Diff and Delta commands in MidiModel; fix marshalling of notes to avoid float->int conversion of length+time properties; initial implementation (not tested much so far) of different policies for how to handle note overlaps

git-svn-id: svn://localhost/ardour2/branches/3.0@7254 d708f5d6-7413-0410-9779-e7cbd77b26cf