- 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 default window title in the designer file wasn't a good string
to translate. Use one of the titles instanced from elsewhere.
- The window titles set in subclasses were not translatable.
FIXES#1092
