Changing parameters of list properties fails

The actual class a list property has depends on the settings for "multiple select" and "relational storage", choosing between StringProperty, StringListProperty and DBStringListProperty.

When changing the values for "multiple select" and "relational storage", the existing objects are not correctly updated, so trying to change them in any way results in exception (see XWIKI-299 ). This means that saving the document is not possible, and deleting the document leaves some properties in the database, which will later exist if the document is re-created.