Enhance clang-tidy configuration and fix remaining warnings

**Describe the bug**
The current clang-tidy configuration should be improved as it

- still produces warnings that should be fixed
- should be configured through a dedicated configuration file rather than hosting its config in a CMake variable

Both issues are more of a clean-up measure, but should improve both, compilation output readability (and conciseness) and clarity of the current configuration. Many issues stem from system headers, too - this should possibly fixed by ignoring them entirely.

**To Reproduce**
Just compile in a clean environment - many issues will be shown.

**Expected behavior**
No remaining clang-tidy issues (if possible) when doing a clean compile.