Add a new API to allow clients to register a custom GFile implementation
handling a particular URI scheme.
This can be useful for tests, but also for cases where a different URI
scheme is desired to be used with another custom GFile backend.
As an additional cleanup, we can use this to register the "resource" URI
scheme too.
Based on a patch by Jasper St. Pierre <jstpierre@mecheye.net>.
https://bugzilla.gnome.org/show_bug.cgi?id=767887