Fix deprecated QRegExp

Use QRegularExpression instead.

https://doc.qt.io/qt-5/qregularexpression.html#notes-for-qregexp-users

This function was introduced in Qt 5

sources/diagramprintdialog.cpp

@@ -299,7 +299,9 @@ void DiagramPrintDialog::updatePrintTypeDialog()
 		{
 			if (pdf_choice_ -> isChecked() && filepath.endsWith(".ps"))
 			{
-				QRegExp re("\\.ps$", Qt::CaseInsensitive);
+				QRegularExpression re
+					("\\.ps$",
+					 QRegularExpression::CaseInsensitiveOption);
 				filepath.replace(re, ".pdf");
 				filepath_field_ -> setText(filepath);
 			}