Make adding user tools possible.

Adding new categories is not there yet though, neither is moving tools
through the categories.

src/plugins/coreplugin/dialogs/externaltoolconfig.h

@@ -68,6 +68,7 @@ private slots:
     void revertCurrentItem();
     void updateButtons(QTreeWidgetItem *item);
     void updateCurrentItem();
+    void addTool();
 
 private:
     Ui::ExternalToolConfig *ui;