Fixed: Delegate method fires twice when using keyboard to select rows in...

... table view

Previously, navigating through the rows of a tableview with the arrow keys would cause the delegate method `- (BOOL)tableView:(CPTableView)aTableView shouldSelectRow:(int)rowIndex` to fire twice. This was due to the delegate method being called twice in checking whether the row could be selected.

This commit changes it so that the delegate method is called just once and the result stored.