WIP V3: fix moving tracks repaint bug

When a header row moved down to make room for a track row,
the column spanning is now reset on the now-track row.

app/playlistmodel.py

@@ -651,6 +651,11 @@ class PlaylistModel(QAbstractTableModel):
             # Optimise: only add to map if there is a change
             if old_row != new_row:
                 row_map[old_row] = new_row
+                if self.is_header_row(old_row):
+                    # Reset column span
+                    self.signals.span_cells_signal.emit(
+                        old_row, HEADER_NOTES_COLUMN, 1, 1
+                    )
 
         # For SQLAlchemy, build a list of dictionaries that map plrid to
         # new row number: