Use a gradient sky to improve visibility in shaded areas,
while also helping the user orient themselves. The bottom of the sky
is black, while the top of the sky is white.
This change also makes it so that the Default Clear Color project setting
no longer affects the Advanced Import Settings dialog.
Adds a bool import option `nodes/import_as_skeleton_bones`.
This is supported in all FBX or GLTF document based formats.
It is especially useful for retargeting and importing animations.
