[FEM] activate spring constraint

- the spring constraint is currently unused but it can be used for Elmer mechanical analyses
This PR does so and also overhaul the dialog UI.
- use the spring constraint also in an example file

src/Mod/Fem/App/FemConstraintSpring.h

@@ -36,8 +36,9 @@ class FemExport ConstraintSpring : public Fem::Constraint
 public:
     ConstraintSpring();
 
-    App::PropertyFloat normalStiffness;
-    App::PropertyFloat tangentialStiffness;
+    App::PropertyStiffness NormalStiffness;
+    App::PropertyStiffness TangentialStiffness;
+    App::PropertyEnumeration ElmerStiffness;
     App::PropertyVectorList Points;
     App::PropertyVectorList Normals;