cubicweb #3406713 Handle the 'placeholder' of input text and textarea [not validated]

StringField should accept a 'placeholder' parameter.

prioritynormal
typebug
done in3.20.0
load1.000
load left0.000
closed by#927d035b6921 [formfields] Handle 'placeholder' attribute for text and textarea
patch[formfields] Handle 'placeholder' attribute for text and textarea. [applied]