In editor indication of unique status of resource

[Zireael07]

Operating system or device - Godot version:
Win 7 x64

Issue description:
Toggling Make unique on a resource in editor has results that are very much not obvious (the only way to tell is to carefully watch the resource name - i.e. press Edit after toggling make unique).

Can we please get an output message telling us the current status? Even better, the resource might have some sort of an icon in the inspector indicating the uniqueness.

Steps to reproduce:
See description above

Link to minimal example project:

[akien-mga]

Some visual info about the uniqueness would be nice to have indeed.

[mhilbrunner]

@akien-mga This is a duplicate of #6922.