This patch adds two items to the dataengine: "Next identifier suffix" and "Previous identifier suffix" that says where to search the previous and next comics (if any) so now it's not tied to a date and to the concept of having one update a day.

and this fixes the xkcd comic (bug 153872) that uses integer identifiers instead of dates for the comic so now the logic to where search the next/previous is delegated to the comicprovider, and if there is no suffix the dataengine simply loads the today's comic.

the cache still works, but is disabled for today's comic, because it can become stale if it was actually an old comic and a new one is published today, and of course today it won't have a "Next identifier suffix" but tomorrow will.