Multiple-threads loading (#425)

Add Multiple-threads loading demo

loading/multiple_threads_loading/preload.gd

@@ -0,0 +1,33 @@
+extends Node
+
+
+var Queue
+
+
+func _ready():
+	# Initialize
+	Queue = preload("res://resource_queue.gd").new()
+	# Call after you instance the class to start the thread.
+	Queue.start()
+
+
+func _process(_delta):
+	# Returns true if a resource is done loading and ready to be retrieved
+	if Queue.is_ready("res://main.tscn"):
+		set_process(false)
+		# Returns the fully loaded resource
+		var next_scene = Queue.get_resource("res://main.tscn").instance()
+		get_node("/root").add_child(next_scene)
+		get_node("/root").remove_child(self)
+		queue_free()
+	else:
+		# Get the progress of a resource
+		var progress = round(Queue.get_progress("res://main.tscn")*100)
+		get_node("ProgressBar").set_value(progress)
+
+
+func _on_Button_button_up():
+	get_node("Button").hide()
+	set_process(true)
+	# Queue a resource
+	Queue.queue_resource("res://main.tscn", true)