XRToolsStaging: warns if no Fade is added as a child

XRToolsStaging requires a MeshInstance3D to hold a transition
fade. This adds a new property so the user is warned
to add this node to staging.

Fix issue GodotVR#629

addons/godot-xr-tools/staging/staging.gd

@@ -284,12 +284,12 @@ func load_scene(p_scene_path : String, user_data = null) -> void:
 ## unless hidden.
 func set_fade(p_value : float):
 	if p_value == 0.0:
-		$Fade.visible = false
+		fade.visible = false
 	else:
-		var material : ShaderMaterial = $Fade.get_surface_override_material(0)
+		var material : ShaderMaterial = fade.get_surface_override_material(0)
 		if material:
 			material.set_shader_parameter("alpha", p_value)
-		$Fade.visible = true
+		fade.visible = true
 
 
 func _add_signals(p_scene : XRToolsSceneBase):