Allow setting no default account

This allows the user to select an account to use every time they launch
an instance.

logic/lists/MojangAccountList.cpp

@@ -93,9 +93,16 @@ MojangAccountPtr MojangAccountList::activeAccount() const
 void MojangAccountList::setActiveAccount(const QString& username)
 {
 	beginResetModel();
-	for (MojangAccountPtr account : m_accounts)
-		if (account->username() == username)
-			m_activeAccount = username;
+	if (username.isEmpty())
+	{
+		m_activeAccount = "";
+	}
+	else
+	{
+		for (MojangAccountPtr account : m_accounts)
+			if (account->username() == username)
+				m_activeAccount = username;
+	}
 	endResetModel();
 	onListChanged();
 }

logic/lists/MojangAccountList.h

@@ -122,6 +122,7 @@ public:
 
 	/*!
 	 * Sets the given account as the current active account.
+	 * If the username given is an empty string, sets the active account to nothing.
 	 */
 	virtual void setActiveAccount(const QString& username);