pat-select: re-visit how "data-value" is calculated

[pysailor]

Inspired by https://github.com/quaive/ploneintranet/issues/4296
There, we have a <label> with pat-select. The label contains a <select> and after it, two hidden <input> fields. The data-value that pat-select puts on the <label> is taken from the last hidden field, instead of from the selected option of the <select>.

Check if the algotithm that computes this can be improved. E.g. disregard hidden fields?