Allow class_name to also add a description

[xDGameStudios]

Godot version:
3.1.alpha.calinou.4c1a5d9

OS/device including version:
OSX High Sierra

Issue description:
Not really and issue just a "request"!! to my knowledge there is no way of adding a description to a class or resource when using class_name (keyword). It is possible to add an icon but no description, that would be a good (and simple, I think) addition.

[Zylann]

This could be related to #6204 and #20318 in the sense of providing metadata to the editor for scripts and their contents, and IMO it's part of the (actually many) little "meta" things people have been requesting to script classes and members.

[willnationsdev]

Yeah, I agree. This is a job for annotations (and really, the custom icon should be too, possibly even the class_name keyword itself as well).

[Xrayez]

Given the idea with annotations in GDScript 2.0 and implemented node descriptions in #31389, is this feature redundant?

[Calinou]

@Xrayez Not completely, as we'd like the description to show up in the Create New Node dialog.

Since implementing godotengine/godot-proposals#177 or another documentation generation proposal would address this, I'll close this issue.