drop use of bounding box to determine whether an item covers a point; add Item::covers(Duple const&)

Default implementation for Item still uses bounding box, but specializations for Arc (Circle), Polygon, Line and PolyLine have been added

libs/canvas/canvas/poly_line.h

@@ -31,6 +31,8 @@ public:
 	PolyLine (Group *);
 
 	void render (Rect const & area, Cairo::RefPtr<Cairo::Context>) const;
+
+        bool covers (Duple const &) const;
 };
 	
 }