Add cppcheck to CI/CD

[zjeffer]

To improve code quality, performance, and avoid bugs, we could run cppcheck: http://cppcheck.net/ and check for improvements (similar to clang-tidy).

I've tested it out on the repo with the following options:

cppcheck --enable=all --inconclusive --force --inline-suppr --library=qt --project=build/compile_commands.json --suppress="*:3rdParty*" --suppress="*:build/*" -i 3rdParty/ -i build --suppress=missingIncludeSystem --suppress=missingInclude --quiet -j$(nproc) --disable=unusedFunction --check-level=exhaustive 2>&1 | tee cppcheck.txt

Some of the suppressions are because we don't care about 3rdParty code, and because some false positives are popping up (missing includes when they aren't actually missing).

Here's the output: cppcheck.txt

Cppcheck can be integrated into PRs with GitHub actions so that when a user submits a PR, a cppcheck bot will 'review' the PR and leave comments to improve the code. We should also run it periodically to check for improvements.

[nuttyartist]

Sounds good!