ToxManager: added approveFriendRequest: method; FriendRequestsVC: adding "add friend" button
Refactoring: renaming: ToxFriendsManager -> ToxFriendsContainer
ToxManager: removing friendRequest after adding friend
FriendRequestsVC: handling ToxFriendsContainer notifications
ToxManager, ToxFriendsContainer: refactoring
ToxManager: loading friends on start
ToxManager: adding new friend after approving request; FriendVC: handling notifications; refactoring