[Development] Add functionality to QPdfWriter

[Erik Janssens]

Hi,

We would like to add some functionality QPdfWriter to be able to
embed custom xmp metadata and attachments in the pdf files generated
by QPdfWriter.

This would improve the handling of pdf files created in Qt by other
applications, such as document managment systems.

The proposed changes are documented in :

QTBUG-78651
QTBUG-78764

Would such changes be welcome, and would a pull request with
such changes have a chance of being accepted ?

An alternative solution to the same problem would be change the API
somehow that the individual pdf objects generated by QPdfWriter can
be retrieved.
The developer would then be able to re-assemble his own pdf mixing
objects generated by Qt with other things.
Such an approach however seems much more work and more convoluted.

First time post, so any feedback is welcome.

Best regards,

Erik