XRToolsStaging: Should warn if no Fade is added as a child

[esodan]

XRToolsStaging should warn if no $Fade, a MeshInstance3D using a QuadMesh, hasn't been added as a child.

The warning should be shown and a new property fade hasn't been set by the user. Also, reference to $Face should be replaced with this new property

[Malcolmnixon]

The script is intended to be used in conjunction with the provided staging.tscn scene file which includes the fade mesh. Instantiating it separately would require the developer to manually create many nodes - or for the script to programmatically construct those nodes.

Unfortunately Godot currently adds all named scripts to the "Add Child Node" wizard; making it difficult to prevent people from instantiating new nodes rather than the configured scenes.