Allow to retrieve only rules that are not inherited

Right now the API to save rules only allow to save explicitely the rules in the xobjects. And the API doesn't allow to add new rules: the javadoc specifies that all rules are replaced, and to add rules one need to first get the rules, add a new rule, and save all the rules.
The problem is that there's no way to distinguish between a rule inherited, and a rule that was explicit. So right now you might end up saving a rule explicitely while it was previously inherited from the space or wiki.
It could be a problem if you want some rules to be still handled by the administrator, at wiki level, so you only want to save the rules that are not inherited. A simple way for fixing that is to provide an API to allow retrieving only rules that are not inherited and to rely on them for saving.