PDF: add support for PDF/X-4

PDF/X-4 is a subset of PDF 1.6, aimed at printing fidelity. We can
support it with a few refactorings of the existing code in
QPdfEngine.

* Add the new PDF version to QPagedPaintDevice / QPdfEngine.

* Always write the XMP metadata, no matter what's the PDF version
  used. XMP used to be written only for PDF/A-1b, but it's supported
  by PDF 1.4 and 1.6 so there's little reason not to write it.

* While at it, ditch the search&replace approach for the metadata
  and use QXmlStreamWriter instead, since it gives us extra
  flexibility that we need (emit different tags depending on the
  PDF version in use).

* The old code had a bug where the timestamps in the XMP metadata
  and the document information dictionary could fall out of sync.
  Just use one datetime object in both places.

* Add /ModDate and xmp:ModifyDate (required).

* Add the required attributes in the xmpMM namespace.

* Add a way to set the document ID to a custom UUID, and use it
  in the XMP metadata as well as in the /ID in the trailer. Emit
  the ID unconditionally, as it's been available since PDF 1.1.

* Emit the output intent for both PDF/A-1b and /X-4. This will be
  amended in a future commit to let the user choose the colorspace.

The only missing bit is §6.5.4 of the PDF/X-4 spec. This imposes that
all symbolic TrueType fonts shall *not* specify an Encoding, and have
exactly one encoding in the cmap table. This is basically requiring what
§5.5.5 in PDF 1.6 only suggests (page 400). However it seems that we are
not embedding a cmap table when extracting a font subset, and that's
already violating PDF/A-1b anyhow. This is tracked by QTBUG-125405.

This work has been kindly sponsored by the QGIS project
(https://qgis.org/).

[ChangeLog][QtGui][QPdfWriter] Support for PDF/X-4 has been
added.

Pick-to: 6.8
Task-number: QTBUG-125405
Change-Id: Ia81f29b07b819eca5767c9f17692d92a3010f5ad
Reviewed-by: Allan Sandfeld Jensen <[email protected]>
Reviewed-by: Volker Hilsheimer <[email protected]>

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