Implement saving account list to file

Currently it only saves when accounts are added or removed. We'll have
to fix this, but we need signals for when account objects change first.

MultiMC.cpp

@@ -150,6 +150,7 @@ MultiMC::MultiMC(int &argc, char **argv) : QApplication(argc, argv)
 	// and accounts
 	m_accounts.reset(new MojangAccountList(this));
 	QLOG_INFO() << "Loading accounts...";
+	m_accounts->setListFilePath("accounts.json", true);
 	m_accounts->loadList();
 
 	// init the http meta cache