Fix for IE7 checkbox input fields

On IE7 checkboxes can only be checked from JS after
the Node has been added to the DOM.

see http://stackoverflow.com/a/579156
Fix widget_name derivation

The form prefix and the parent prefix do not necessarily contain a
trailing dot. Use z3c.form.util.expandPrefix to account for this.
jQuery UI 1.9+ compatibility

jQuery UI 1.9 unified the naming convention for data keys. Use the new
keys by default, but add a check for the old key value to retain
compatibility with Plone 4.2.
Attach dropdown menu to widget instead of the body

This makes the dropdown visible if the widget is in an overlay.
Add correct CSS class to the widget div

The somewhat unfortunate derivation from QuerySourceXXXWidget makes all
this a bit harder than necessary. Maybe the widget should be derive from
MultiWidget instead.
Add "template" for newly added items