Cleanup of bone animation code

Слиты xottab-duty запросил слияние github/fork/TmLev/refactor-animations-bones в xd_dev

Created by: TmLev

This pull request aims to:

  • Add appropriate attributes for some methods.
  • Use nullptr where possible instead of 0 / NULL.
  • Adapt range-based for.
  • Add override and const where appropriate.
  • Remove starting _ in some names (because they are reserved by the compiler).
  • Clean up commented out code and unused definitions.