Game over screen + big fixes

When any game ends (including custom ones) a new game over screen is
shown. This screen includes a scrollable list of all of the words you
made for that game and various statistics; highest value word, longest
word, most used word, number of unique words and total number of words.
Also fixed numerous bugs throughout the system, including a structural
bug regarding the array _gridLetters. Also fixed the back buttons not
working in the menus, among other things.