Deprecated Title::userIsWatching(); use User::isWatched() instead.

Deprecated Title::userIsWatching(); use User::isWatched() instead.

* The problem is that Title::userIsWatching() relies on $wgUser,
  which is not suitable on every case. Instead User::isWatched()
  requires both an User and a Title object.
* Replaced all core calls from the former to the latter
* Added a cache in User for the WatchedItem instances so we do not
  need to do a database request every time something want to know
  whether a page is watched or not, which can happen several times
  per request.

Change-Id: Ifa9c55b7ffb487ce6893c74df233eedc7654dc5e