CMake: Allow CMake configuration to be forced to string type

This allows editing values with misidentified types in Project Mode.

Change-Id: Ic74da2ca71cc9046cbbeb1202075976c9edd28b7
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildsettingswidget.h

@@ -66,6 +66,8 @@ private:
 
     void updateSelection(const QModelIndex &current, const QModelIndex &previous);
 
+    bool eventFilter(QObject *target, QEvent *event);
+
     CMakeBuildConfiguration *m_buildConfiguration;
     QTreeView *m_configView;
     ConfigModel *m_configModel;