gui: ipc: Support cancellation of preview generation

gui/ipcserver.h

@@ -4,14 +4,19 @@
 #include <QLocalServer>
 #include "ipc.h"
 #include "filesaver.h"
+#include "ipcpreviewworker.h"
+
 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();