add MusicRoom.add action keyword argument

I added keyword argument "action" to MusicRoom.add.

                def add(self, filename, action=None, always_unlocked=False)
                `action`
                    This is a action or the list of actions. these are called when this
                    file is played.

                    For example, These actions is used to change a screen or background, description
                    by the playing file, and allowing multiple pages by single MusicRoom instance.