A few fixes to interpolation of MIDI controller data. Don't interpolate

A few fixes to interpolation of MIDI controller data.  Don't interpolate
when writing these data back to a source, otherwise surprising new
interpolated points appear in MIDI automation.  Similarly don't interpolate
when reading the model during MIDI stretch.  Fix handling of interpolation state;
controllers that have been set by the user to use a different interpolation style
are noted in the <Source> tag of the session file and this state is sprayed around
to MidiModel and the GUI as necessary.

git-svn-id: svn://localhost/ardour2/branches/3.0@7409 d708f5d6-7413-0410-9779-e7cbd77b26cf