Clean up outdated comments

[hoffie]

• 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.