Use :ty rather than :path for ffi_name and ffi_class_name

[ids1024]

I believe this is more correct, and should allow more things to work without breaking anything. But it's hard to be sure with complicated macros...

This allows code like this to compile:

glib_wrapper! {
    pub struct SimpleObject(
        Object<(<SimpleObjectPrivate as ObjectSubclass>::Instance),
        (<SimpleObjectPrivate as ObjectSubclass>::Class), SimpleObjectClass>)
        @extends gtk::Widget;

    match fn {
        get_type => || SimpleObjectPrivate::get_type().to_glib(),
    }
}

[sdroege]

As everything apparently still compiles, let's get this in. The reason why this was needed is apparently not valid anymore :) Thanks for noticing this!