file-backend: avoid deadlock when portal op is canceled

Calling g_cancellable_disconnect() inside a cancelled handler is a guaranteed deadlock. Cancellables should only be canceled once, so we don't need to worry about a second cancellation occurring. I think it's sufficent to disconnect when the InitClosure is freed.

Possibly fixes #86 (closed), perhaps, hopefully?

Warning: this is speculative and not tested at all