Fix MIDI CC iterator infinite looping.
Only allocate Text widget for MIDI events if necessary.
git-svn-id: svn://localhost/ardour2/branches/3.0@3307 d708f5d6-7413-0410-9779-e7cbd77b26cf
* make MidiModel keep track of its CanvasProgramChange instances
git-svn-id: svn://localhost/ardour2/branches/3.0@3297 d708f5d6-7413-0410-9779-e7cbd77b26cf