Description : Data types containing a Data Reference to a C1 Page have previously used a drop down for selecting pages. On sites with many pages this UI element have been very un-friendly.
A Form UI XML tag have also been added to the default form ui namespace, which can be invoked like this:

      <DataReferenceTreeSelector Label="Select page" Help="Behold the new selector!" Handle="Composite.Management.PageIdSelectorDialog" DataType="Composite.Data.Types.IPage,Composite" NullValueAllowed="true">
        <DataReferenceTreeSelector.Selected>
          <cms:bind source="PageId" />
        </DataReferenceTreeSelector.Selected>
      </DataReferenceTreeSelector>

New data types created will automatically use this widget, and existing data types can have their widget (or form UI xml) changed to use it.