[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/Gui/TaskFemConstraintDisplacement.cpp

@@ -489,7 +489,6 @@ std::string TaskFemConstraintDisplacement::get_zFormula() const
     QString zFormula = ui->DisplacementZFormulaLE->text();
     zFormula.replace(QString::fromLatin1("\""), QString::fromLatin1("\\\""));
     return zFormula.toStdString();
-    ;
 }
 
 bool TaskFemConstraintDisplacement::get_dispxfix() const