[IMP] web: allow fields in list view to use the attrs readonly

With this you can use filters such as `attrs="{'readonly': [('other_field', '=', False)]}"` in list views

Steps to reproduce:

  1. modify sale.order.line tree view (in so form) and add for the field 'name'  attrs="{'readonly':[('product_id', '!=', False)]}"
  2. select a product

Observed:
 The content of the description field is hidden

Expected:
The description field is readonly
The old code does not seem to consider `this.datewidget.$input` possibly being `null`, why does the new code do so?