file_class should have a way to access the fuse object

file_class instance may need to access the main fuse object for state information (e.g. database connection, network connection etc.). But the nested class doesn't seem to have any easy way to do that. This is the case in example/xmp.py. And there doesn't seem to be an alternative way to use the stateful file API.

I can certainly just add the file handle at the last argument of file callbacks. That's what I eventually did.