Core: Return context help id by callback

...to support asynchronous providers.

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

src/plugins/cmakeprojectmanager/cmakeeditor.h

@@ -38,7 +38,7 @@ class CMakeEditor : public TextEditor::BaseTextEditor
     Q_OBJECT
 
 public:
-    QString contextHelpId() const override;
+    void contextHelpId(const HelpIdCallback &callback) const override;
 
     friend class CMakeEditorWidget;
 };