client: Make CmCallback work like other glib callbacks

Instead of passing the user_data as first argument, pass it as last argument like anywhere else. Instead we have the client as first argument as one would expect.

Remove mentions of user_data being a GObject requirement. It's an untyped pointer as user_data elsewhere.

This is a slight API break but it makes things less confusing (at least for me). As I always get stuck on gpointer object and similar things.