Gui dashboard

Gui dashboard (actually only contains leaderboard now, but future work will include more dashboard like functionality where the player can see the status of his/her city and perform some tasks).

The Peaderboard class is derived from a generic Popup class, that we could use for other things (credits roll, login, what have you). Last thing to mention is that i put some medals in there, to see what you guys think about it.

The leaderboard needs a nested array as data, and exposes an 'update' function for realtime updating.