Enable autoload in editor

[vnen]

• Tool scripts will be executed and can be accessed by plugins.
• Other script languages can implement add/remove_named_global_constant
  to make use of this functionality.

Edit: some more notes:

• The autoloads will actually be in the editor tree, so calling get_node("/root/autoload") will work as well.
• Editing the autoloads in the project settings dialog will update the tree and the constants immediately.
• Behavior in-game will remain exactly the same, and there will be no API to add/remove autoloads on the fly there.

Closes #4236.

[Ranoller]

I can´t find any way to disable this feature in project settings, Is there any way? Will be autoloads ever running game code in editor? Actual behaviour is nonsense, preloads runs over the editor, action_events are executed inside the editor, etc... etc... etc... is like have the game running (To the point that i can´t move the editor screen without the keys because my game doesn´t allow that)

[akien-mga]

Please file a bug report, if the feature is misbehaving the solution is not to disable it in project settings, it's to fix it.

[vnen]

@Ranoller Code should not run in editor unless it is a tool script. But please open an issue about it and I'll take a look later.