Shift+Drag of clips between tracks was never allowed up to 2.4.2...

... and in 3.0.0 started crashing instead.  Just disallow it again, no
functionality lost.

Signed-off-by: Sol Fisher Romanoff <sol@solfisher.com>
Signed-off-by: Panagiotis Vasilopoulos <hello@alwayslivid.com>

src/tracks/ui/TimeShiftHandle.cpp

@@ -622,6 +622,10 @@ namespace {
       TrackList &trackList, Track &capturedTrack, Track &track,
       ClipMoveState &state)
    {
+      if (state.shifters.empty())
+         // Shift + Dragging hasn't yet supported vertical movement
+         return false;
+
       // Accumulate new pairs for the correspondence, and merge them
       // into the given correspondence only on success
       Correspondence newPairs;