Use GSource dispose function for safely disconnecting the GCancellable source cancelled signal handler

If not doing this it might happen that the cancelled signal is emitted
between reaching a reference count of 0 and finalizing the GSource, at
which point part of the GSource is already freed and calling any GSource
functions is dangerous.

Instead do this from the dispose function. At this time the GSource is
not partially freed yet and calling any GSource API is safe as long as
we ensure that we have a strong reference to the GSource before calling
any GSource API.