Before it was possible that changes for media files the user can't
access were shown if the user could access a page with the same name.
This also reverts the file existence check so it now really checks for
media changes that the media file exists and for page changes the page
and that not one of the two suffice for both.

As we need to keep all core meta files anyway (the only core file that
had been deleted but shouldn't be deleted is the .indexed file) and
plugins can keep care of their own metadata files using the
IO_WIKIPAGE_WRITE event there is no reason for using the expensive
metaFiles() function during the deletion of pages.

Sometimes pages aren't deleted from the index (that should be fixed,
too) and appear in the backlinks although they don't exist anymore.
This change hides them and hidden pages that shouldn't appear at all as
the backlinks view is an automatic listing. Hidden pages had been hidden
before the transition to the metadata index, too.