Improve integration of tool Nodes and AutoLoad singleton objects

[drtwisted]

1. Create scene
2. Add a Node
3. Add a simple tool script to it

tool
extends Node

export(Texture) var player_sprite
export(int) var health_points

4. The exported vars are visible in the Inspector
5. Create unattached script global_singleton:

extends Object

func some_global_method():
    print('Called global method')

6. Add it to AutoLoad scripts in Project Settings
7. Append next code to your Node script:

func some_method_using_singleton():
    global_singleton.some_global_method()

Actual result:
The exported vars are gone from the editor and the Node is in fact no longer a tool

Expected:
The Node tool mode persists

[Zireael07]

Is that on master?

[27thLiz]

The exported vars are gone from the editor and the Node is in fact no longer a tool

That's because the script fails compiling, since autoloads can't be used in tool scripts yet.
Closing as duplicate of #4236.