Support events triggered from a wiki targeting document of another wiki

Our current event API makes very ambiguous the distinction between the wiki where an event is triggered, and the wiki where the document targeted by the event is stored.

Basically the second information has never been stored when we were using the DB storage for events. It's now stored in Solr events, but it's not necessarily easy to use it, and the event API does not allow to retrieve the information. 

See some investigation on the problem in https://jira.xwiki.org/browse/XWIKI-19400?focusedCommentId=111368&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-111368