sorting: Allow adding sorting clauses to an existing GomSorting

This allows another way to use GomSorting objects.

In addition, it makes it possible to use GomSorting from GI-based
bindings.

As it turns out, GObject Introspection does not support variadic
functions, so gom_sorting_new just isn't usable from GI-based bindings.

But with this commit, GI-based bindings can now simply create a new,
empty GomSorting object, then add their sorting clauses to it.

https://bugzilla.gnome.org/show_bug.cgi?id=752141