Clean up outdated comments

- Remove commented out code as it is confusing. It will still be part of the git history if needed again.
- Check if `TODO`'s are actionable
- If they are, open issues for them and remove them.
- If they are, but they are deprecation reminders, keep them.
- If they aren't, drop them.
- Check if old compatibility code can be dropped (`AT LEAST QT 4.8 IS REQUIRED`?)
- Check "temporary workaround"-style comments. Check if the workaround can be removed or remove the comment if the workaround has proven to stay required.

Search the repository for commented-out code, TODOs, compatibility notes, and temporary-workaround comments. Review each occurrence to determine whether it is obsolete, actionable, a deprecation reminder, or still required. Done means obsolete material is removed, actionable TODOs have corresponding issues, and necessary reminders or workarounds remain documented.

Written by the indexing model from the issue text.