[ScreenSelectMaster] Added GetCurrentPageNumber() Lua binding.
comments for future changes to ScreenSelectCharacter to bring it in line with more modern screens
[ScreenSelectCharacter] Replace hardcoded references to 320 with SCREEN_CENTER_X. It'd be better if the themer could determine where these were actually placed, but that's for another time.
Merge branch 'master' of github.com:shakesoda/stepmania