pdf preview generation: Use QtConcurrent::mapped + QFutureWatcher instead of own single-thread solution

gui/mainwindow.h

@@ -32,7 +32,7 @@ class MainWindow : public QMainWindow
 	QSqlDatabase db;
 	QFuture<QVector<SearchResult>> searchFuture;
 	QFutureWatcher<QVector<SearchResult>> searchWatcher;
-	PdfWorker *pdfWorker;
+	QFutureWatcher<PdfPreview> pdfWorkerWatcher;
 	void add(QString path, unsigned int page);
 	QThread searchThread;
 	QThread pdfWorkerThread;