- point to main Calamares site in the 'part of' headers instead
of to github (this is the "this file is part of Calamares"
opening line for most files).
- remove boilerplate from all source files, CMake modules and completions,
this is the 3-paragraph summary of the GPL-3.0-or-later, which has
a meaning entirely covered by the SPDX tag.
The (renamed) class TranslatableString keeps a key value
(e.g. New_York) and a human-readable version around; the
human-readable one is passed through QObject::tr() for translation
on-the-fly.