Rename small and large tile to sub- and megatile

Primarily intended to remove all_the_fourteen_underscore_method_names,
but it also sounds cooler and it kinda makes more sense. Subtile because
that's pretty much what they are. Megatile because mega3t and
sup(er)tile would look too much like subtile. Also renamed all
occurrences of "small area" by megatile because that makes more sense.
"Someone won a small area" was weird, cause what's a small area? Someone
won a megatile sounds *way* cooler and makes more sense.
Draw lines across won subtiles
Highlight won megatiles in winner's color
UI changes and such for quit/restart/game over