Editor plugin: Disable editation when signature plugin is active and vice versa

Pdf4QtEditorPlugins/EditorPlugin/editorplugin.h

@@ -52,6 +52,7 @@ public:
     bool save();
 
 private:
+    void onSceneActivityChanged();
     void onSceneChanged(bool graphicsOnly);
     void onSceneSelectionChanged();
     void onWidgetSelectionChanged();