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
Very many testcases, some GLib tools (resource compiler, etc) and
GApplication were calling g_type_init().
Remove those uses, as they are no longer required.
https://bugzilla.gnome.org/show_bug.cgi?id=686161