Separate slider ball and followcircle skinnables into default/legacy classes

[goodtrailer]

Continuation of #14832 based on comments from #18795

The original PR was too ambitious. As mentioned, the changes should have been more granular. This is the first part of the original PR and aims to make improvements to the project's file structure, which is necessary to implement the slider anim changes. However, keeping with the idea of granularity, this PR is meant to be treated mostly individually, as this PR is intended to be beneficial on its own, whether or not the slider anim changes are accepted.

Summary

• Separates slider ball and followcircle skinnables into Default and Legacy classes
• Moves game logic from SliderBall into new DrawableSliderBall class
• Renames fullSizeFollowCircle to followCircleReceptor

Rationale

• Separating slider ball and followcircle into Default and Legacy classes:
  • Better fits the already in-place format that other skinnables follow (spinners, approach circles, etc.)
  • Provides more flexibility w.r.t. animating slider balls and followcircles
• Moving game logic into new DrawableSliderBall class:
  • Classes in the Skinning namespace should pertain to visuals, not to actual game logic (i.e. tracking, ball movement)
  • Necessary for splitting slider ball into Default and Legacy (without duplicating an ungodly amount of code)
• Renaming fullSizeFollowCircle to followCircleReceptor:
  • Better reflects what the object actually does (acts as an invisible, virtual hitbox)
  • Is similar to how DrawableHitCircle names its own hitbox

[peppy]

Class is a bit pointless but I guess it's okay.