Add the possiblity to create custom top level nodes in the scene tree.

[sebavan]

Currently, we can customize the inspector with plenty of flexibility and add some custom properties and such on existing types like meshes and the like of it.

The overall idea would be to allow the creation of a node at the same level than Textures and Materials on which we could display our custom properties.

It would prevent the need of creating for instance an empty mesh to add custom properties on it.