Selection: tolerate empty (False) name

This shouldn't have happened at the server side, but we can still be
tolerant.
ButtonFieldWidget: evaluate button's context attribute

This happens when the button needs to customise the context of its
action:
   <button name="action_name" context="{'default_foo': my_foo}" />

The string context shall be evaluated against the current record's
fields.
ActionFieldWidget: give better feedback if action is not found
KooModel: mention my copyright, add a comment