Call updateValue before returning value in DateTime-field -> getValue()

This is needed because value of datetime-field is returned from internal
variables not directly from input-field and therefore needs to be updated
before returning.
Find parent with instanceof instead of xtype-compare

This enables to override parent (kwc.newsletter.recipients) in web.
Was needed for TIS to add a custom action to filter recipients.
Merge pull request #235 from Ben-Ho/3.7-newsletter-stuff

3.7 newsletter stuff