No way to select class items in theme

[QbieShay]

Godot version: 3.1 custom build

Issue description:
Themes allow different Items for each class but there is no way to select them. For example, I have a Label with 3 different fonts and I would like to be able to select the font that fit best in that situation.

Steps to reproduce:

1. Make a theme
2. Make a class (Label)
3. Make a class item (font)
4. Make another class item (font)
5. Try to apply one of the two fonts in the editor

[Chaosus]

Seems like #20320, but not sure whether it a duplicate or not..

[YuriSizov]

If I understand correctly, Chaosus is right that this is a duplicate of #20320 and therefore of godotengine/godot-proposals#850. Both of which, I believe, are covered by this PR: #47544.

[YuriSizov]

#47544 and #50169 make it possible to define variations of theme classes and pick them from a list in the Inspector (and set them via code too if needed).

I don't think that in the foreseeable future we will be able to pick from a list of available items on a single class, so this is the solution that we have for this problem. So I'll close this.