Simplification and better comments relating to time-shift...

... fewer switches on USE_MIDI.  In fact capturedClipArray is nonempty
whenever capturedClip is not null.

src/Snap.h

@@ -40,8 +40,10 @@ public:
 
    Track *track;
    Track *origTrack;
-   WaveTrack *dstTrack;
    WaveClip *clip;
+
+   // These fields are used only during time-shift dragging
+   WaveTrack *dstTrack;
    std::shared_ptr<WaveClip> holder;
 };