lib/configuration improvements

Configuration changes:
- Configuration::value return type is now [[nodiscard]]
- Configuration::value<T> is now a generic template that only works with
the exact types of the underlying std::variant
- Add Configuration::value<concept_value_t> for standard library types
compatible with the types of std::variant
- Add Configuration::shortcut<> placeholder, and QAction and
QKeySequence specializations as a convenient way to set up shortcuts
- Deprecate setShortcut
- Add Configuration::read_file convenience member that takes file path
as parameter

Format changes:
- Configuration files can now have sections, specified as [section
name]. Section names are prepended to keys. Section names cannot be
nested.
- Configuration files can now have @@include directives, causing another
file to be read as well. The included file is not treated as nested into
a section, and will overwrite values previously set.

Others:
- add some tests for libconfiguration. QAction/QKeySequence require a
QApplication be set up, so the test application may require running
xorg/wayland.

old coverage: lines: 15.6% (960 out of 6172) branches: 9.9% (1187 out of 12012)
new coverage: lines: 17.1% (1067 out of 6254) branches: 11.0% (1388 out of 12644)

0 files changed, 0 insertions, 0 deletions