Feature #3906 / Mark required inputs with an asterik

Implemented by using a new class ''.required-field'. The class has to be applied to the label. Not implemented is the option, to render the asterix directly on/beside the input (not the label) - probably solved later.
qfq-bs.css.les: New CSS class required-field
AbstractBuildForm.php: extended buildLable() with new parameter $addClass.
BuildFormBootstrap.php: Assign class 'required-field' to label for required fields.