dplib: move most of DPLobby to a new DPSession class

[goto-bus-stop]

A DPSession will represent a single game session, and the DPLobby class will map to DirectPlay's own lobby stuff with some sugar sprinkled on top.

Currently:

auto lobby = new DPLobby(new DPGameAoC(), "Player name");
lobby->host();
lobby->launch();

Maybe-future:

auto lobby = new DPLobby();
auto game = new DPGameAoC();
auto session = lobby->hostSession(game);
session->launch();

// maybe?
auto session = lobby->joinSession(game, address, sessionGuid);
session->launch();

[goto-bus-stop]

544c884897351d743c6bf36320cef069e706b10c