Add schema for saving controls

[Geokureli]

A compact way to serialize and deserialize the input mappings of a FlxControl instance

1. controls.saveMappings: Returns a string representing that input layout
2. controls.loadMappings: Takes a mapping string and replaces the current mapping, entirely
3. mappings constructor arg: Prevents creation of the default mappings and goes straight to the mappings provided
   • How will this work with multiple players sharing a keyboard?