backgroundManager: Always emit 'loaded' signal

As backgrounds are cached, it is possible that we never emit the
'loaded' signal added in commit f386103b. We are relying on the
signal though, so do the same as Background and emit the signal
from an idle if the background was already loaded.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1371


(cherry picked from commit 5c550dae)