current selection is cleared if popup menu opened on region with no groups

Obscure problem noticed when testing JAL-2374 patch.

1. Load the example alignment (feredoxin.fa)
2. create a group on the alignment (columns 21-50)
3. Click drag to mark columns 10-19
4. Right click an unselected and ungrouped region (say second row, column 5)

Nothing happens - the popup menu is not shown because there is no selection under the mouse. (this is correct)

5. now adjust the horizontal scroll bar to move the rubber-banded region off screen and then back on. Notice the rubber-banded region disappears, but the column markers stay. The sequence ID panel may also refresh to show that now there are no sequences selected.

Two implications of this behaviour:
* trashing the selection appears to be a bug, since the viewport doesn't update.
* its probably not desirable to trash the users current selection because they right-clicked somewhere else. For a large alignment with thousands of groups this can also result in a slowdown because Jalview has to search all the groups to see if they intersect the region that was clicked.