Bug501: Improve time-shift of clips between tracks...

... When you click in a stereo clip, move the correct partner clip with it.

When you click in the selection and it spans clips, move all clips containing
any part of the selection.  (As horizontal dragging does.)  Even if these are
not all within the same stereo track.

I implemented this rule:  If any clip moves up (or down) by n audio tracks
(not counting the label tracks), all clips must move the same number, and
all must move from mono to mono or left to left or right to right.  If that
is not possible, no move happens.

src/Snap.h

@@ -34,10 +34,12 @@ public:
    TrackClip(Track *t, WaveClip *c)
    {
       track = origTrack = t;
+      dstTrack = NULL;
       clip = c;
    }
    Track *track;
    Track *origTrack;
+   Track *dstTrack;
    WaveClip *clip;
 };