Unlike the last two commits, we don't port GimpConfig to
G_DEFINE_INTERFACE(), since we need to provide a base initializer
for it.  However, this commit aligns it with the rest of our
intefaces, by renaming gimp_config_interface_get_type() to
gimp_config_get_type(), and by performing class initialization for
the interface in a separate function than base initialization.

Keep gimp_config_interface_get_type() around as a deprecated
function, to maintain ABI compatibility.  It will be removed in a
separate commit in master, so that this commit can be easily
cherry-picked to gimp-2-10.