CMake: Only enable edit button for CMake configuration when useful

Only enable the button when items are actually editable.

Change-Id: I54fd5430772a6db61c85b59d5c655d3bd4ebf8d7
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildsettingswidget.h

@@ -64,6 +64,8 @@ private:
     void updateAdvancedCheckBox();
     void updateFromKit();
 
+    void updateSelection(const QModelIndex &current, const QModelIndex &previous);
+
     CMakeBuildConfiguration *m_buildConfiguration;
     QTreeView *m_configView;
     ConfigModel *m_configModel;