Allow selection extension in leoEditCommands.backToHome.
Two more collapses in complexity:
1. Key handlers now set vc.return_value = False to indicate that the key has not been fully handled.
   This eliminates *all* return values from handlers!
2. Began the process of eliminating all separate visual mode motion handlers.
   They will be replaced by the corresponding normal-mode motion handlers,
   which will extend the selection if vc.state == 'visual'
   This will eliminate a huge amount of duplicate code.