Core: Return context help id by callback

...to support asynchronous providers.

Change-Id: I483489c74e7886d5bc2bf00b65540c3d2c7afee0
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmldesigner/components/texteditor/texteditorwidget.h

@@ -50,7 +50,7 @@ public:
         return m_textEditor.get();
     }
 
-    QString contextHelpId() const;
+    void contextHelpId(const Core::IContext::HelpIdCallback &callback) const;
     void jumpTextCursorToSelectedModelNode();
     void gotoCursorPosition(int line, int column);