Sketcher: GeoId, GeoElementId and GeoUndef refactor

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This commit is an independent refactor of the identifications used at Sketcher level.

It introduces a new type "GeoElementId" as a combination of GeoId and PointPos.

It moves the Undefined GeoId, previous Constraint::GeoUndef to GeoEnum, together with all
other fixed values of GeoIds.

src/Mod/Sketcher/App/SketchObjectPyImp.cpp

@@ -1372,7 +1372,7 @@ PyObject* SketchObjectPy::calculateAngleViaPoint(PyObject *args)
 
 PyObject* SketchObjectPy::isPointOnCurve(PyObject *args)
 {
-    int GeoId=Constraint::GeoUndef;
+    int GeoId=GeoEnum::GeoUndef;
     double px=0, py=0;
     if (!PyArg_ParseTuple(args, "idd", &GeoId, &px, &py))
         return 0;