bluetooth: Do not sync() immediately on model changes

bluetooth: Do not sync() immediately on model changes

Our intended behavior when bluetooth is turned off is to keep
the menu visible if devices had been set up previously.

However since gnome-bluetooth@c437c729, devices are removed
first before removing the default adapter, so we now end up
always setting the property to false before checking for it.

Fix this by deferring all model changes to an idle, so that
we can process them as a unit. Do the same for proxy property
changes, as those may trigger a row-removal.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1174