Fix nuxeo-input placeholder default styling

Currently, placeholder text of nuxeo-input is of the same color as input text. The placeholder text should be styled with a lighter shade to differentiate and convey that the element has no input (see screenshot for reference).

Steps to Reproduce:
1. In a Web UI layout (e.g. file creation layout), update a nuxeo-input element with placeholder text, for example:

<nuxeo-input
      role="widget"
      label="[[i18n('title')]]"
      name="title"
      value="{{document.properties.dc:title}}"
      placeholder="Write your File name here..."
      autofocus
      required
    >
</nuxeo-input>

2. Navigate to the form in Web UI and observe the updated nuxeo-input element.

Expected behavior: placeholder text is lighter than input text

Actual behavior: placeholder text is of same styling as input text