fix "fail to build from source" with Qt < 5.7, and in the same occasion fix a memory leak

git-svn-id: svn+ssh://svn.tuxfamily.org/svnroot/qet/qet/trunk@5424 bfdf4180-ca20-0410-9c96-a3a8aa849046

sources/diagramview.h

@@ -56,6 +56,7 @@ class DiagramView : public QGraphicsView
 		QPointF           m_rubber_band_origin;
 		bool              m_fresh_focus_in,
 						  m_first_activation = true;
+		QList<QAction *>  m_separators;
 	public:
 		QString title() const;
 		void editDiagramProperties();