Allow empty slots as targets for moves

Also, buttons that would target invalid positions will now be not only disabled but also hidden.

How it will look now:
Bisharp using Brick Break: http://i.imgur.com/MxHMb3L.png
Gastly using Shadow Ball: http://i.imgur.com/cLrJ9YQ.png
Gengar using Helping Hand: http://i.imgur.com/ISoHcpC.png
Gengar using Shadow Ball: http://i.imgur.com/xd8zq07.png
Talonflame using Acrobatics 1: http://i.imgur.com/s8nXBFc.png
Talonflame using Acrobatics 2: http://i.imgur.com/hIhb1ll.png