DateField: allow specifying a default pre-selected date in the picker without setting a value on the field itself.

[flochtililoch]

Right now, the DateField picker will automatically focus to the date:

• present on the value attribute
• when not already set, the value set in the max attribute
• when neither set, the current date

It could be useful to allow pre-selecting another date that's not coupled to either the value or the max attributes.