We need an easy way to list all empty entities, and perhaps also other "kinds" of entities, on special pages etc. This could easily be done via the page_props table, we just need to start using it. The page_props table is fed from the ParserOutput object (via LinksUpdate).
So, EntityContent::getParserOutput() should just add the desired info to the ParserOutput - it could be provided by a getPageProps() method or some such.
Initially supported "hints" or "kinds" could be:
- empty: contains nothing at all
- stub: consists only of label(s) and description(s) (and maybe also aliases).
There could be more, like "unlinked" for items with no sitelinks.
Whiteboard: u=dev c=backend p=2