add a 'Move Backward' menu option; rename selectNextSelection to selectN...

...extWindow for consistency

Having a Move Forward but no Move Backward (only a Previous Selection toggle) is frustrating.  Functionality for Move Backward already exists, this patch just exposes it in the UI so I can add a keybinding.  Also, renamed a method for consistency.

I gave up fighting xcode to get this to build, so patch is untested; sorry about that. :(