#71 class not deselected properly when added

[graeme foster]

1) Select a class
2) Click and drag so another class is highlighted
3) Allocate the class to the time slot

The first class is allocated.
The other class that was "dragged over" is still
highlighted.

Try and allocate again an error message is displayed in
the status bar.

[graeme foster]

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user_id=1356778

Just added the method:

void TT_Generation::clearMLASelection()
{
ui.artLectures->clearSelection();
ui.artLabs->clearSelection();
}

and called it from TTSlotClicked() when a slot is added.