Outline: Add context menu for collapsing/expanding tree

Task-number: QTCREATORBUG-2976

src/plugins/cppeditor/cppoutline.h

@@ -17,6 +17,8 @@ class CppOutlineTreeView : public Utils::NavigationTreeView
     Q_OBJECT
 public:
     CppOutlineTreeView(QWidget *parent);
+
+    void contextMenuEvent(QContextMenuEvent *event);
 };
 
 class CppOutlineFilterModel : public QSortFilterProxyModel