on_page_add should have access to the newly created page

When the `on_page_add` event is called, it should pass in a reference to the *new* page instead of to `$this` (which in my case resolved to the "drafts" system page). Otherwise there is no way to get to the newly created page, since not even the cID of the new page is stored...
Merge pull request #1332 from infostreams/patch-1

on_page_add should have access to the newly created page