Form elements: Provide possibility to visually hide the label

[mfranzke]

We should even also provide the possibility to visually hide the label, as there might be edge cases, in which this would be necessary. These cases might include that we accept a user target group that doesn't need visual labels, very simple cases in which even the WCAG makes exception like e.g. search field. But we should as well add a disclaimer for this option to not have it used too frequently.

see also db-ui/core#585