Make FlxKey dedicated to identifying keys. Closes #1164

In this change:
* `FlxKey` is no longer a `FlxInput`.
* `FlxKeyInput` typedef is now used instead of the old `FlxKey` for input monitoring.
* `FlxKey` is now an abstract enum mapping enum names and strings to keycodes.
* No need for `keyLookup` anymore. Strings automatically cast to `FlxKey`s.
* String mapping is now case-insensitive.