Add scene property to inlined glTF examples

[javagl]

The examples (for example, the minimal glTF file at https://github.com/KhronosGroup/glTF-Tutorials/blob/master/gltfTutorial/gltfTutorial_003_MinimalGltfFile.md ) are supposed to be minimal and complete files.

As of KhronosGroup/glTF-Sample-Models#242 , these examples should all have a default scene property. The examples from the tutorial should basically correspondond to the "simple sample models", and it should be possible to copy+paste+view these samples as standalone files.