graphics/darktable: Update to 4.2.0

Release notes:
https://github.com/darktable-org/darktable/releases/tag/release-4.2.0

The work was done by Matthias Andree (mandree@) in
`graphics/darktable42`. Some of the style fixes were added in a separate
commit. Thank you Matthias!

The temporary port `graphics/darktable42` can be removed at the same
time, now that the main port is up-to-date.

Submitted by:	mandree@