Added 19x19 ladder support

This new release adds some support to the 19x19 ladder in DGS.
The client now downloads the 19x19 DGS ladder once and displays your rank as well as the list of players that you can challenge. This information is stored in the client, so that you don't have to download again the ladder (which is time and bandwidth consuming) the next time you want to see it. But of course, you can also download again the ladder on-demand.
You can also challenge one of these players.