Fix the handling of director classes with final methods

Generated SwigDirector_* classes were attempting to override
methods marked as final.

In addition, give a warning if the destructor of a director class is
final.

Closes #564.

Examples/test-suite/python/cpp11_final_directors_runme.py

@@ -0,0 +1,11 @@
+import cpp11_final_directors
+
+class Derived2(cpp11_final_directors.Derived):
+
+    def meth(self):
+        return 3
+
+
+b = Derived2()
+if b.meth() != 3:
+    raise RuntimeError