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/line_set.cc

@@ -104,3 +104,9 @@ LineSet::clear ()
 	_bounding_box_dirty = true;
 	end_change ();
 }
+
+bool
+LineSet::covers (Duple const & /*point*/) const
+{
+	return false;
+}