Use SkinnableDrawables for gameplay objects

[LumpBloom7]

Opens up the opportunity to support different skins later on down the line.

Current list of skinnable elements

• Tap circle
• Hold body
• Slide star
• Slide chevron
• Slide fan chevron
• Touch body
• TouchHold body

With the current way I am using slide fan chevrons (wrapping them in a BufferedContainer, so that alpha looks normal), it would be very expensive for skins that provide a simple version, since that would incur the same overhead. It may be more worthwhile if I fallback to providing several slide chevron sprites, and using that instead.