Disable "Save All" when there are no documents to be saved

Fixes: QTCREATORBUG-22072
Change-Id: I6c84e0004d1ada27bfcec59f509d066f1b03ca2c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/documentmanager.h

@@ -162,6 +162,8 @@ private:
     void checkForReload();
     void changedFile(const QString &file);
     void filePathChanged(const Utils::FilePath &oldName, const Utils::FilePath &newName);
+    void updateSaveAll();
+    static void registerSaveAllAction();
 
     friend class Core::Internal::MainWindow;
     friend class Core::Internal::DocumentManagerPrivate;