GSettingsBackend: emit changes to correct thread

When g_settings_apply() is called on a delayed settings backend and
there is a D-Bus error when communicating with dconf-service, recent
versions of the dconf GSettingsBackend call a function in GLib that
improperly delivered the signal directly instead of using
g_main_context_invoke().

This patch fixes this function to route in the same way as the others so
that the signal is dispatched in the proper GMainContext.