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

gui/pdfworker.h

@@ -6,29 +6,17 @@
 #include <QThread>
 #include <QMutex>
 #include <QWaitCondition>
+#include <QMutex>
+#include <QFuture>
 #include <poppler-qt5.h>
 #include "pdfpreview.h"
 #include "searchresult.h"
+
 class PdfWorker : public QObject
 {
 	Q_OBJECT
-
-  private:
-	QHash<QString, Poppler::Document *> documentcache;
-	Poppler::Document *document(QString path);
-	std::atomic<bool> cancelCurrent{false};
-	std::atomic<bool> generating{false};
-	QMutex isFreeMutex;
-	QWaitCondition isFree;
-
   public:
-	PdfWorker();
-	void cancelAndWait();
-  public slots:
-	void generatePreviews(QVector<SearchResult> paths, double scalefactor);
-  signals:
-	void previewReady(PdfPreview p);
-	void previewsFinished();
+	QFuture<PdfPreview> generatePreviews(QVector<SearchResult> paths, double scalefactor);
 };
 
 #endif // PDFWORKER_H