Fix updating the state of menus.

The menus were only updated during context changes
(not if action states where updated directly),
and didn't take into account that the active action for a context
might still be disabled.
Also, enabled separators should not automatically lead to enabled menus.

src/plugins/coreplugin/actionmanager/command.h

@@ -81,6 +81,7 @@ public:
 
 signals:
     void keySequenceChanged();
+    void activeStateChanged();
 };
 
 } // namespace Core