[Development] Support for *Notes and UpstreamFiles fields in qt_attributions.json files

[Edward Welbourne]

Hi all,

Having taken part in various third-party updates and felt a need to
leave notes for those who will do the same in future, I have run up
against JSON not having a comment format.  To work round that, I propose
to allow some fields to be included in a qt_attribution.json file for
that purpose.  As a general pattern, I propose allowing ${Field}Notes
for any ${FIELD} that already exists; and a separate UpstreamFiles
field, corresponding to the Files one, to list where in the upstream
source tree to find the files that are to be copied.

This will make the work easier for those who do third-party updates and
avoid various kludges presently in use to work round the lack of
comments (for example, abusing an existing field to first write the note
and then over-write it with the value; the tools that process the files
cope, but it's malformed JSON).

See:
* https://codereview.qt-project.org/c/meta/quips/+/460358
* https://codereview.qt-project.org/c/qt/qttools/+/460116
* https://codereview.qt-project.org/c/qt/qttools/+/460181
* https://codereview.qt-project.org/c/qt/qtbase/+/458824

The last triggered this; the previous two implement it and the first is
an update to QUIP 7.

	Eddy.