Fix bunch of issues for B-splines (#3155, #3156, #3157, #3159)
Issue #3155: Increase low boundary of Weight for Bspline (minimal positive value of B-spline weight is set)
Issue #3156: Prohibit selection of Bspline in equality constraint (Equal, Tangent and Perpendicular constraints are protected by validators)
Issue #3157: Sign of Tangent constraint between ellipse arc and Bspline is shown directly on connection point
Issue #3159: Equality and Intersection constraints are enabled for B-spline
