Cppcheck github action, and style and bugfixes

[HybridDog]

I've used Cppcheck to find and fix some bugs and bad code style, and then added a github action to run the checks automatically. I haven't tested if the github action works.

The bugs:

• AddonPreviewMenu::rebuild_menu accessed the addon description string out of bounds.
• TextObject::wrap_text stripped all newlines, even double ones, because prev_c was not changed.
• RainParticleSystem::set_amount used amount instead of real_amount in the minimum calculation, which was probably not intended. I don't know how this affects the rain.

Related issue: #1552 In the future when code is more clean, additional linters could be added, e.g. TscanCode and Clang-Tidy. SuperTux apparently already has a .clang-tidy configuration file but no github action workflow for it.

[HybridDog]

I think the new workflow works now and the 5 failing checks are not related to this PR and fail on other PRs, too.

[HybridDog]

Thank you for reviewing this. Can I merge it now?

[mrkubax10]

I'm not sure if I should decide about this but I think that you can

[HybridDog]

CONTRIBUTING.md and the wiki don't mention conditions for merging (at least I cannot see them), so I assume that I'm allowed to merge.