Use relative instead of global paths in resources

[Jummit]

Describe the project you are working on

An addon.

Describe the problem or limitation you are having in your project

Since addons can be put into subfolders now (godotengine/godot#43734), I put downloaded plugins in res://addons/thirdparty for organization. This breaks most plugins though, as they use global paths, for example res://addons/plugin/resource.tres.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

If a resource is stored inside a file (.tscn, .tres, etc), the paths should be saved relative to the file.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Inside res://addons/plugin/test.tscn:

test.png -> res://addons/plugin/test.png

If this enhancement will not be used often, can it be worked around with a few lines of script?

You can manually replace the paths with local ones, but they get overridden once you save the resource.

Is there a reason why this should be core and not an add-on in the asset library?

It's core.

[Jummit]

I realize new that this is kinda a duplicate of #2375 and #2327, closing. The only difference is that this proposal says it should be default.