Remove Glib.Boxed usage in Value.
Remove Glib.Boxed usage in Value.

* glib/Value.cs: GLib.Boxed is obsolete and removed.

Gankov/gtk-sharp,antoniusriha/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,akrisiun/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,akrisiun/gtk-sharp,antoniusriha/gtk-sharp,akrisiun/gtk-sharp,orion75/gtk-sharp,antoniusriha/gtk-sharp,antoniusriha/gtk-sharp,akrisiun/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp