Update AnimationBuilder and stuffs in `combinators`

[Multirious]

• Move combinator/state module into a file.
• Move all animation combinators into combinator/animation_combinators module.
• Add private time_runner_or_default method to reduce boilerplate.
• Add more (forgotton) methods to AnimationBuilder
  • skipped
  • disabled which combines skipped and paused
  • time_scale
  • direction
• Update CHANGELOG.md and also include changes from previous non PR commits.
• Update docs for some combinators