Splines can now be created, only control points can be set, because degree parameter doesn't have any influence on the spline, it seems to be 2.
A new button is created when selecting spline, to end the spline creation.
std::vector had to be declared as list for lua-intf tu use std::vector required for Spline constructor.
Each points in the Lua table have to be copied individually each time the mouse move, as Lua copy the table by reference and not value.

Screenshot of spline creation: