Matrix Editor Improvements (editing multiple segments)

   Just pushed some improvements to editing of multiple segments in the 
Matrix Editor courtesy of Mark Rubin.  Changes include:

   - Clearer display of the notes in the active segment.

   - Notes can now be drawn in the active segment over top of notes
     in the inactive segments.

   - Improvements to the selection of notes when editing multiple
     segments.  Only notes from the active segment are selected.

   - Velocity tool only affects notes in the active segment.

   If you use the Matrix Editor to edit multiple segments, please test 
latest git.

Ted.