Create example for WebEngine's Print To Pdf

Adds example command line tool 'html2pdf' that loads a URL into
QWebEnginePage and prints it to a pdf file.

Task-number: QTBUG-59823
Change-Id: I898e9a9ecdf819f417876f964d40e494f6095c99
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>