Datepicker: need support for scoping themes

Using the datepicker with some of the other widgets

on the same page leads to conflict when both use

different themes.

For example, let we have an accordian menu and a form

with datepicker, both in the same page.

If we want to set a different theme to the datepicker,

the only way is to download a custom theme using the

ThemeRoller with a "CSS Scope" option e.g. ".theme_datepicker".

But the UI generated datepicker div uses an absolute position, so

it can't use the custome ".theme_datepicker" class and that's

why we can't set a different theme to the datepicker.