Crossfading of mixins

[JakobKallin]

Crossfading between scenes, as requested in issue #38, is now implemented, but mixins cannot yet be crossfaded. Instead, playing a mixin with fading simply mixes in the new scene immediately.

[JakobKallin]

Note that this could become void with a simpler and more flexible scene model, as mixins might not be needed to begin with.