placeholder attribute not supported in textarea widgets

[frisi]

the placeholder attribute is not supported by HTMLTextAreaWidget (HTMLTextInputWidget supports it)

is this by intent or would a PR be welcome?

i'd suggest add it to HTMLTextAreaWidget and include it in textlines_input.pt and textarea_input.pt so devs can use placeholder text if they want to.

What I did:

set the widgets placeholder in a plone dexterity schema

    widget(
        "contact_persons",
        placeholder="Firstname, lastname, role",
    )

    contact_persons = schema.Tuple(
        title=_("Contact Persons"),
        value_type=schema.TextLine(),
        required=True,
    )

What I expect to happen:

the textarea in the resulting form should show
"Firstname, lastname, role" as placeholder text

What actually happened:

no placeholder text is shown

What version of Python and Zope/Addons I am using:

plone 5.2.4 - z3c.form 3.7.1

[icemac]

Sounds reasonable, a PR is welcome.