Redesign Frame Duration

[AnonymouX47]

• Redefines Renderable.frame_duration.
  • Removes None as a possible value.
  • Now raises NonAnimatedFrameDurationError when invoked (GET) on non-animated renderables.
• Redefines RenderableData.duration.
  • Removes None as a possible value.
  • Now left uninitialized for render data generated by/for non-animated renderables.
  • The value range and meaning is now "unspecified" for non-animated renderables.
• Redefines Frame.duration.
  • Removes None as a possible value.
  • The value range and meaning is now "unspecified" for non-animated renderables.

Quick branch off #100