Removed DatetimeField->setDescription() proxying

We shouldn't pass it to the child fields since it ends
up showing the description three times in the default
"inline" mode. If the description is required as a hover/focus effect,
it can be set individually on the fields.
Removed widths on DatetimeField children

Makes it more flexible in regards to field widths,
e.g. DMY fields vs. a single input field. Also allows
to extend with additional fields more easily, e.g.
a drop down through the "timedropdownfield" module.