Introduce some Calamares helpers in the compat/ headers
space to help Xml-wrangling (i18n support tools, GeoIP)
avoid compiler warnings about deprecated API.
It's cleaner this way anyway, with a nice value returned
from setting a document's contents, rather than a bunch
of pointer arguments.
- 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.
