GH-1726 better failure detection for updates

Instead of just checking if the new version started, make sure
it is able to write its IPC key to a file and then use the key
to connect to the process.

libraries/LocalPeer/src/LocalPeer.cpp

@@ -108,6 +108,11 @@ ApplicationId ApplicationId::fromCustomId(const QString& id)
 	return ApplicationId(QLatin1String("qtsingleapp-") + id);
 }
 
+ApplicationId ApplicationId::fromRawString(const QString& id)
+{
+	return ApplicationId(id);
+}
+
 LocalPeer::LocalPeer(QObject * parent, const ApplicationId &appId)
 	: QObject(parent), id(appId)
 {