backends: rate-filter the emission of MetaBackend::last-device-changed signal

It indirectly triggers expensive operations in gnome-shell
(js/ui/keyboard.js), which turns out too expensive if we happen to operate
the shell simultaneously with 2 devices that will trigger the operations
there.

So just rate limit the signal emission, defer to an idle and just emit
the last device gotten. Worst that will happen is that we may possibly
emit the signal on the same device consecutively.

https://bugzilla.gnome.org/show_bug.cgi?id=753527