Sketcher: Constraint Third Redefinition for internal geometry

=============================================================

Internal alignment of BSplines requires an extra index for unbounded same-type internal geometry (poles, probably also in the future knots).

Value shall not be used, as it interferes with the update mechanism for constraint values.

An alternative solution would be to add another int to all the constraints just for this case.

As internal geometry does not use Third, it seems reasonable to reuse it for this indexing purpose.

src/Mod/Sketcher/App/Constraint.h

@@ -110,7 +110,7 @@ public:
     PointPos FirstPos;
     int Second;
     PointPos SecondPos;
-    int Third;
+    int Third; // Note: for InternalAlignment Type this index indexes equal internal geometry elements (e.g. index of pole in a bspline), thirdpos remains unused. It is not a GeoId. 
     PointPos ThirdPos;
     float LabelDistance;
     float LabelPosition;