Add widgets API for printing on a QPrinter

Renders the content to a PDF document and then renders this on a
QPrinter-backed QPainter using the PDFium library.

PDFium bitmap to QImage conversion based on work by Paulo Pinheiro
<paulvap.os@gmail.com>.

[ChangeLog][QtWebEngineWidgets][Printing] Enables printing QWebPage
content on a QPrinter. Currently does not support previewing the
document. Widgets only for the moment.

Change-Id: I9a5264433093379aee90f5e4f69bf2aee8814f2b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>