(Romance,Por,Spa) refactor Extend{Por,Spa} into ExtendRomanceFunctor

- couldn't get a functor on top of another working in GFSS2018, so now
  we just copy the relevant definitions from ExtendFunctor and add the
  Romance-specific ones
- add stare_V to DiffRomance (and essere_V for symmetry) so that we
  can use it in ExtendRomance; using Latin verb name for fairness;
- add two missing lins to ExtendFunctor

  + related changes

src/romance/DiffRomance.gf

@@ -189,4 +189,9 @@ oper
     <RPres,Simul>  => <verb ! VFin (VPres m) n p, []>
     } ;
 
+  oper
+    essere_V : Verb ;
+    stare_V : Verb ;
+    stare_V = essere_V ;
+
 } ;