- 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.
- These methods are used for multi-page view-steps, which are rare.
For all the others, just drop the empty implementation and defer
to the base class.
In order to avoid deep copies, Qt containers over which we iterate must be const
The remaining unported cases require qAsConst (Qt 5.7) or std::as_const (C++17)
