css_class value not propagates to subfields of DateTime form

Hi dev's,

 recently there has been a bug report that the "css_class" value is in
fact ignored when rendering DateTime fields:

 http://sourceforge.net/mailarchive/forum.php?thread_id=3547899&forum_id=1702

 Indeed the problem is that the value needs to be propagated to the
subfields embedded in the DateTime field.

 The bug can be fixed via a simple "on_value_css_class_changed" helper;
however on the formulator-general list I have been uncarefully discussed
the option of having different css-classes for the date and time values.

 As nobody came up with a real use case for this I guess there is no use
case for it.

 So if nobody objects on this list I will check in the simple fix
sometime near the end of the next week (after I found the times to write
tests for it :-> )

Cheers,
Clemens