Merge branch 'relative_snap'

      - Many changes to the "User Interaction" pane wrt key modifiers.
      - Snap is now relative by default (override to absolute with the
        "Snap to absolute using:" modifier).
      - Midi notes now obey both snap modifiers (disable snap and absolute snap)
      - Timefx drag now selects the primary region (less user confusion).
      - Includes MIDI event id fix.