Improve mega selection in randoms.

Mega evolutions now have their own viable moves selected for random sets (this improves Heracross, Mewtwo and Charizard).  Additional changes to mega movesets to be more viable are probably needed, but I don't know which moves would be viable.

On random team generation, once a mega evolvable pokemon is selected, other potential mega evolvable candidates will be selected as regular pokemon (that is, if Gengar with Gengarite was already added to the team, Heracross with Choice Scarf can still be added now).