Godot: Visually linking exported Nodes in the editor

[leoddd]

See #6015.

Being able to connect two nodes visually in the editor would help immensely with level design clarity and game logic bug tracking.
It's a lot more obvious to every level designer when a lever is connected to the wrong door with a line than when it's hidden as a property that only shows up when clicked on, to give a really simple example.

I think https://github.com/godotengine/godot/issues/6015#issuecomment-241388278 had a really good idea of how to expose this functionality through export parameters, since exporting a NodePath is the only realistic usecase I can think of where linking would be useful:

export(NodePath, "#FFFFFF") var attack_entity = @""

Optionally a few line styles could be supplied as well, such as solid, dotted, dashed etc. to give the user more control over indicating what each connection means. A dotted line could indicate which entity a lever is attached to, while dashed lines could show warp destinations, where a white line is a warp for the player and a red line is a warp for enemies, etc.

Example of how it could look by @bojidar-bg:

[eon-s]

It can be easily done with tool mode (even with a helper custom node), not sure if will be good to add for this kind of specific use case.

[akien-mga]

Am editor plugin as @bojidar-bg had started to work on would do the trick IMO.

[leoddd]

A plugin would work of course, but I feel like wanting to visually connect two nodes is a thing that pretty much every game that exports a NodePath to the editor could make use of, and I'm not actually sure how one would configure such a plugin to show the right kind of connection style if one can't use the export hints for that.

[Calinou]

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