scale combobox: regenerate pdfs preview with choosen scale

gui/mainwindow.h

@@ -47,6 +47,7 @@ class MainWindow : public QMainWindow
 	void showSearchResultsContextMenu(const QPoint &point);
 	void tabChanged();
 	void pdfPreviewReceived(PdfPreview preview);
+	void comboScaleChanged(QString text);
 };
 
 #endif // MAINWINDOW_H