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/lists/MojangAccountList.h

@@ -161,10 +161,9 @@ protected:
 	QList<MojangAccountPtr> m_accounts;
 
 	/*!
-	 * Username of the account that is currently active.
-	 * Empty string if no account is active.
+	 * Account that is currently active.
 	 */
-	QString m_activeAccount;
+	MojangAccountPtr m_activeAccount;
 
 	//! Path to the account list file. Empty string if there isn't one.
 	QString m_listFilePath;