Offline mode support, part 1

Refactor MojangAccount so it exposes a less generic interface and supports login. Hide the ugly details.
Yggdrasil tasks are now only used from MojangAccount.

logic/LegacyInstance.cpp

@@ -105,7 +105,7 @@ MinecraftProcess *LegacyInstance::prepareForLaunch(MojangAccountPtr account)
 #endif
 
 		args << "-jar" << LAUNCHER_FILE;
-		args << account->currentProfile()->name();
+		args << account->currentProfile()->name;
 		args << account->sessionId();
 		args << windowTitle;
 		args << windowSize;