background: properly disconnect background signals

background: properly disconnect background signals

BackgroundManager connects to the changed signal in
the backgrounds it manages.

The signal ids for the changed signal connectionss are stored
as state on the background manager object.

If the background being managed changes while the manager
is still loading the old background, then the signal id
variable can get out of sync with the background object being
managed.

This commit ties the signal id to the background objects themselves,
so there is no opportunity for them to desynchronize.

https://bugzilla.gnome.org/show_bug.cgi?id=696157