gui: ipcworker,ipcserver: Refactor

Crashes were observed, faulting in libQtNetwork.
Those were rather rare. We also have no traces.

Probably depends on some order signal/slots were
processed. Remove shared states between connections,
such as the IPCPreviewWorker and socket instance in IPCServer.

gui/ipcserver.h

@@ -10,13 +10,10 @@ class IpcServer : public QObject
 {
 	Q_OBJECT
   private:
-	IPCPreviewWorker previewWorker;
 	QLocalServer spawningServer;
-	QLocalSocket *currentSocket = nullptr;
 	SaveFileResult addFile(QString file);
   private slots:
 	void spawnerNewConnection();
-	void handlePreviewGenerated(QByteArray ba);
 
   public:
 	IpcServer();