Fix clip left behind after two move operations

The undo helper only saves info for tracks that were affected.
So we can not merge two move operations if they affected different
tracks. Maybe in the future we can merge the undo helpers.

As reported here:
https://forum.shotcut.org/t/beta-version-24-02-now-available-to-test/43064/18

src/commands/timelinecommands.cpp

@@ -721,6 +721,9 @@ bool MoveClipCommand::mergeWith(const QUndoCommand *other)
             || that->m_ripple != m_ripple || that->m_rippleAllTracks != m_rippleAllTracks
             || that->m_rippleMarkers != m_rippleMarkers)
         return false;
+    if (that->m_undoHelper.affectedTracks() != m_undoHelper.affectedTracks()) {
+        return false;
+    }
     auto thisIterator = m_clips.begin();
     auto thatIterator = that->m_clips.begin();
     while (thisIterator != m_clips.end() && thatIterator != that->m_clips.end()) {

src/commands/undohelper.h

@@ -39,6 +39,10 @@ class UndoHelper
     void recordAfterState();
     void undoChanges();
     void setHints(OptimizationHints hints);
+    QSet<int> affectedTracks() const
+    {
+        return m_affectedTracks;
+    }
 
 private:
     void debugPrintState(const QString &title);