Allow attaching an xobject or an attachment to a translation page

I think the problem is that there's a single attachment store per document. In other words, there's currently a single xwikiattachment table with one row per document id, and default documents and translated documents share the same document id (there's just the language column which is different between both). So we'd need to add a language column to the xwikiattachment table.

Regarding xobjects, I think that's a duplicate of XWIKI-69