[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
Can't this be removed entirely? The old code was so that when a field is set to readonly (by the form view's `attrs` computation) the list view forces it to invisible (to show the list view field behind it). There's no point in setting to `readonly` a field which is *already* `readonly`.