0028631: Modeling Algorithms - Crash while building a 2D circle tangent to another circle and passing through its center

Code correction includes also:
- extending GccEnt to have GccEnt_Position conversion to string value and backward;
- new GeometryTest_DrawableQualifiedCurve2d DRAW object to wrap GccEnt_QualifiedCirc/GccEnt_QualifiedLin.
  It is a DrawTrSurf_Curve2d with GccEnt_Position visualized in orange color.
- qcircle/qline DRAW command to create instance of GeometryTest_DrawableQualifiedCurve2d
- circ2d3Tan DRAW command to wrap GccAna_Circ2d3Tan taking qcircle/qline/point arguments.

src/GeometryTest/GeometryTest.hxx

@@ -39,6 +39,9 @@ public:
   //! defines curve commands.
   Standard_EXPORT static void CurveCommands (Draw_Interpretor& I);
   
+  //! defines tangent curve commands.
+  Standard_EXPORT static void CurveTanCommands (Draw_Interpretor& I);
+
   //! defines fair curve commands.
   Standard_EXPORT static void FairCurveCommands (Draw_Interpretor& I);