Add a script that allows to update a class on all wikis

We can have a script that allows to mass update a class on all the wikis, like add a property to the class or modify and existing property.

This can turn to be very useful in the case of upgrades, for XWikiPreferences, if we always keep current preferences (which is a good idea), we might want to update the class to the new form though.
Note that this script is especially useful for XWikiPreferences because of bugs like XWIKI-11874 and XWIKI-11873 .