Add `override` keyword to some methods

I can't see these tickets any longer. Sadly I don't know of any linting
tools that could check the entire codebase for the missing `override`,
thus only the three reported classes are fixed and not more.

Fixes #1001938
Fixes #1001937
Fixes #1001936
Merge pull request #685 from sschaef/t-add-override

Add `override` keyword to some methods