Provide an display method to obfuscate sensite value before making them public

Currently, when returning values that should not be rendered unaltered to the user (e.g., password hash, mails...), each place where it is required to do so needs to write it's own custom code.

It would make the task easier to provide a dedicated display in the various *PropertyClass classes.

The forum discussion: https://forum.xwiki.org/t/obfuscate-xobject-values-before-making-them-public/11845