XWIKI-10110 - Add WatchList macro

- now waits for the "delete item from watchlist" to vanish after
  deleting a watchlist item - should avoid flickering the test due to
  instantly leaving the page after clicking the delete link, possibly
  stopping the delete request before it even reaches the server
- also the various "isWatched" methods now never wait for the
  delete link to show up somewhere, making the test much faster