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/include/LocalPeer.h

@@ -54,8 +54,11 @@ public: /* methods */
 	static ApplicationId fromTraditionalApp();
 	// ID based on a path with all the application data (no two instances with the same data path should run)
 	static ApplicationId fromPathAndVersion(const QString & dataPath, const QString & version);
-	// fully custom ID
+	// custom ID
 	static ApplicationId fromCustomId(const QString & id);
+	// custom ID, based on a raw string previously acquired from 'toString'
+	static ApplicationId fromRawString(const QString & id);
+
 
 	QString toString()
 	{

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)
 {