Row drag and drop action should be treated as a single action in the Alignment Editor

Scenario:

1. Open "data/samples/CLUSTAW/COI.aln".
2. Click on the first sequence name, do not release the mouse button.
3. Drag the sequence name to the last position in the Name List.
4. Release the mouse button.
5. Click "Undo" button on the toolbar.
   Expected result: the row is moved back to the first position, "Undo" button is disabled.
   Actual result: the row is moved to the position before the last one, "Undo" button is still enabled.

Each row moving is treated as a separate action that could be undone. If the mouse button was not released, then it should be treated as a single action.