This commit fixes up a few race conditions in the GSettingsBackend, mostly with
respect to change notifications occuring at the same time as the last reference
count on a GSettings is dropped. With GDBus feeding us our incoming signals in
a separate thread, this is something that could easily happen.
- used in some places as a move-along-as-we-go pointer
- used in other places as a pointer to the fixed base of an array
Switch all users to the first style to avoid a crasher.
