QPdfWriter: Add support for painting in different layers

Suggestion

Perhaps we can add a new function, QPdfWriter::newLayer() which is analogous to QPagedPaintDevice::newPage() ?

Use-case

Description from customer: "Currently, each print job consists of 2 PDF files. One file holds the image and the other one holds the cutting lines. Both files are aligned by defined circles in the PDFs."

"The print shop would like to see both printing and cutting data in a single file. The idea is to have the knife paths in a dedicated layer, or to paint the knife paths using a Spot Color, so that it can be extracted and sent to the cutter."