Use only two GKeyfileSettingsBackends per process
Garnacho says that using a different GKeyfileSettingsBackend for each GSettings instance is causing Epiphany to consume thousands of inotify handles. Uh-oh. This is unnecessary so let's just avoid it.
Probably fixes #865 (closed)
WIP because this depends on unreleased glib 2.61.2.
Edited by Michael Catanzaro