Moved the `FileSettings` out of the `Settings` and pass them around
explicitly. They can never be specified at the same time and are used
exclusively. The code hasn't been fully adjusted to reflect this as this
is only the refactoring without any functional changes.
* added `Settings::useSingleJob()` and use it instead of checking `jobs` or `jointSuppressionReport`
* extracted single job execution into `SingleExecutor`
* moved `reportStatus()` from `CppCheckExecutor` to Èxecutor
* TestSingleExecutor: improved tests
* added testing of markup extension handling in executors
* cleaned up includes based on `include-what-you-use`
* testsingleexecutor.cpp: suppress `performance-unnecessary-value-param` clang-tidy warnings
* ProcessExecutor: send color via pipe instead of applying it beforehand
* do not unconditionally apply colors to output / disable all colors in tests / adjusted tests for changed output behavior
* fixed precision loss in `Executor::reportStatus()`
* fixed `naming-varname` selfcheck warnings
