Allow schema option to define root parent when using the "page" type join

[fotisp]

When defining a join of type "page", it would be useful to provide an option for the developer to provide a "path" under which this page must live under.

Something of type "descendantOf"

To take this issue a step further, it would be ideal to allow the developer to provide a filtering function which appends to the $criteria object, before that is used to retrieved the pages