complete nicknames in last used order
also fixes a couple of other nick completing bugs
this, however, doesn't take into account updateNick
  (see said function in com/ubergeek42/weechat/Buffer.java,
  also the end of com/ubergeek42/weechat/relay/messagehandler/NicklistHandler.java)
 nickname updates should be run on nickname changes and mode changes, but i only see nick additions/removes for this actions, so this works fine in current setup