SPDX KAboutLicense

[Harald Sitter]

Hey!

For general interest, and so this thought doesn't only exist in my
head: As a follow-up to us embracing SPDX for code files it would make
a whole lot of sense sense to move KAboutLicense away from our custom
text format (see KAboutLicense::byKeyword) to SPDX as well. The text
format is particularly used in plugin and plasma applets
metadata/desktop files and while we managed to patch on a semblance of
spdx support by simply hardcoding mapping rules it's super limited
because we are constricted by the actual feature set of KAL, which
among other things means license expressions are not a thing at all
(e.g. [1]) which in turn then limits the license definition I can use
for my plasma applets.

I reckon this can be pulled off backwards compatible, seeing as we
have retrofitted KAL with spdx license tag support we can "up-cast" a
KAL to a SpdxExpression.

Food for thought.

[1] https://phabricator.kde.org/D6672

HS