Simplify and streamline animation classes relationship: wxAnimation is
the only public class representing an animation and it can be created by
both the native wxAnimationCtrl and wxGenericAnimationCtrl using the new
public CreateAnimation() method.
Replace wxAnimationImplType enum with more flexible type info based
check.
This is just unnecessary and having wxAnimation::m_refData->m_refData is
confusing, both in wxGTK version where it's not used and in the generic
one where it is, but can be replaced by more type-safe m_decoder.