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/PropertyConstraintList.cpp

@@ -404,10 +404,10 @@ bool PropertyConstraintList::checkGeometry(const std::vector<Part::Geometry *> &
 bool PropertyConstraintList::checkConstraintIndices(int geomax, int geomin)
 {
     int mininternalgeoid = std::numeric_limits<int>::max();
-    int maxinternalgeoid = Constraint::GeoUndef;
+    int maxinternalgeoid = GeoEnum::GeoUndef;
 
     auto cmin = [] (int previousmin, int cindex) {
-        if( cindex == Constraint::GeoUndef )
+        if( cindex == GeoEnum::GeoUndef )
             return previousmin;
 
         return ( cindex < previousmin )? cindex : previousmin;