Convert license statements to SPDX expressions

Also add/move license files as required by REUSE specification

One license not yet handled is the TWAIN one:

• license document: https://invent.kde.org/graphics/libksane/-/blob/master/COPYING.twain
• file under license: https://invent.kde.org/graphics/libksane/-/blob/master/ksanetwain/twain.h

Any idea how to convert that? And would that need to be converted in the same step, or could this also be done separately at a later point?

@cordlandwehr @sars @pino @jriddell