The scene importer always assumed that the AnimationPlayer is called
"AnimationPlayer".
This is not always true: for example the GLTF importer just creates an
AnimationPlayer with the default name, which may be "animation_player",
depending on the project settings.
This fix instead chooses the first node that is an AnimationPlayer, and
warns if there is more than one.
ec19ed3723
Its a bit broken right now, more patches need to be ported. Also I kept the Transform track for now, and I did not touch the gltf module yet.
