line polygon creation option
Since the polygonisation can fail, we do need a tool so that the user can create a polygon around the line, just as he can create a region. BTW, it would be better to start from a rectangle than from a triangle, as the former is much more current and it is more comfortable in those rare cases where you prefer to have a triangle to go back to a triangle by deleting a point rather than having to add a fourth point for all the normal cases, i.e. regions, or line polygons