The save of the changes is scheduled for a GMainContext, with a chance
to be cancelled when the folder is freed, but it could happen, with
proper timing and thread interleaving, that the dipose() was called
just before the schedule to save the changes was dispatched, but
the dispose() already freed some internal structures, which could
cause a crash elsewhere in the code. This change cancels the scheduled
save as the first thing in the dispose() and it also changes the code
to use a GWeakRef, which ensures the save is scheduled only if
the dispose() was not called yet.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1914917