Allow click-drag to left to select regions in a sequence

Promoted to an improvement.

In Feb 22nd 2016, Chris noticed:
 You can click and drag right to make a selection in a sequence, but dragging left does nothing. NB this is only the case for the initial drag. A selection that is 2 or more residues long can be extended (or shrunk) left or right ok.

Looking at the code in SeqPanel.doMouseDraggedDefineMode()
- the first event is processed within the initial selected residue i.e. before the mouse moves to another column
- 'changeEndRes' is set to 'true' every time (as the condition for 'changeStartRes' is tested second)
- as a result, subsequent drag events are ignored as the test for 'new position to right of selection start' fails

Possible solution:
- save the mouse x-position in mousePressed()
- compare with position in the mouse dragged event to determine drag direction