axes: only update the currentaxes property of the parent figure if the axes object is visible * graphics.cc (F__is_handle_visible__): New function. is_handle_visible (const graphics_handle&)): New function. is_handle_visible (double): New function. is_handle_visible (const octave_valueu&)): New function. * axes.m: Only set currentaxes property in parent and currentfigure property in root if axes object is visible.