Move getAtlasFor and friends from FlxState to FlxBasic

We generate some sprites automatically in the preloader, prior to switching states. As a result, the attempts to call FlxBasic.getAtlas fail, as they forward to a null FlxG.state.

This change moves the atlas management functions up the hierarchy to FlxBasic. This doesn't break FlxState, as it is a direct descendant of FlxBasic.