A bunch of GUI improvements

Buttons that lead you down a "path" where users should no longer be able to mess with the main panel, like the new/continue/view game buttons now replace the main panel instead of creating additional popups. Also, I added a "no custom pieces/variants" label to the respective menu panels so there isn't as much disconnect between how the panel looks with/out items. I also centered the Help and About frames, and did some minor formatting on the Help screen.