disable support for unloading of dynamic types

Experimentally disable the ability to unload dynamic types by refusing
to drop the last reference on types (effectively turning the type
unloading into dead code).

The plan is to leave things like this for a stable cycle and only
proceed with removing the code if we are sure that there are no
unforeseen problems.

https://bugzilla.gnome.org/show_bug.cgi?id=693351

gobject/gtype.c

@@ -2440,6 +2440,14 @@ type_data_unref_U (TypeNode *node,
 		     NODE_NAME (node));
 	  return;
 	}
+      else
+        {
+          /* This is the last reference of a type from a plugin.  We are
+           * experimentally disabling support for unloading type
+           * plugins, so don't allow the last ref to drop.
+           */
+          return;
+        }
 
       g_assert (current > 0);