g-i: Update gidl binding to include type hierarchy
g-i: Update gidl binding to include type hierarchy

The IdlNode* classes should be declared as subclasses of IdlNode, that
way the compiler can be sure that casting from an IdlNode is safe.

frida/vala,GNOME/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala