Allow to specify an xclass reference filter in a page picker

It's currently possible to give the space where to look for pages in a page picker, however it would be handy to be able to specify the reference of an xclass, to filter out all pages that do not own an xobject of this xclass in the results.
Specifically this could be useful for extensions, to suggest results corresponding to a specific page containing an xobject, without having to take care of the space.