Generalize previews: Add PreviewResult,PreviewResultPdf, remove PdfPreview

gui/previewresultpdf.h

@@ -0,0 +1,17 @@
+#ifndef PREVIEWRESULTPDF_H
+#define PREVIEWRESULTPDF_H
+#include <QImage>
+#include "previewresult.h"
+
+class PreviewResultPdf : public PreviewResult
+{
+  public:
+	using PreviewResult::PreviewResult;
+	PreviewResultPdf(const PreviewResult &o);
+	QImage previewImage;
+
+	QWidget *createPreviewWidget() override;
+	bool hasPreview() override;
+};
+
+#endif // PREVIEWRESULTPDF_H