InputCommon: Add a new ExpressionParser to replace the old hack language This contains a new, hand-written expression parser to replace the old hack language based on string munging. The new approach is a simple AST-based evaluation approach, instead of the "list of operations" infix-based hack that there was before. The new language for configuration has support for parentheses, and counts "!" as a unary operator instead of the binary "NOT OR" operator it was before. A simple example: (X & Y) | !B Explicit device references, and complex device names ("Right Y+") are handled with backticks and colons: (`SDL/0/6 axis joystick:Right X+` & `DInput/0/Keyboard Mouse:A`) The basic editor UI that inserts tokens has not been updated to reflect the new language.