Improve handling of custom types

[KoBeWi]

Originally started as a fix for #35205, but I got a bit carried away.

The PR improves how custom types (i.e. types added with EditorPlugin.add_custom_type()) are handled throughout editor:

• added some helper methods to check if custom type exists
• fixed a bug where custom types weren't properly saved in history by create dialog (fixes Custom types can't be favorited #35205)
• custom types will now display their icon in inspector categories
  It looks bad with #57286 though:
  
  but that's another issue to be fixed separately.
• custom types are now recognized by script create dialog. However, since they are not a real types, the script path will be implicitly inserted instead

So about the "they are not a real types" part, I discovered that while going through code. Custom types are supposed to be a helper to add a node with a specific script. They also have a gray icon in scene tree dock, unlike class_name types:

(this should be probably made consistent)
They aren't supposed to be real classes. They don't even register their name anywhere, aside from some internal map in EditorData. I added a mention in the docs that clarifies it a bit.

Their advantage is that you can enable/disable a plugin and the type will be added/removed from editor, but none of your scripts will break, because the type is never referenced by name. Thanks to that they also don't clutter global scope with more class names.
The disadvantage is that you can't create them from script using their name. Also users probably don't expect them to work this way; I bet there are some open issues that are invalid for this reason.

Anyways, this rambling isn't really related to this PR. There were just voices that we should consider removing custom types in 4.0, so the above text could be a good reference for a proposal.

[YuriSizov]

I think this looks fine, besides a small note above. We didn't settle on removing this, so I guess we aren't. Didn't test it, would appreciate a rebase.

Also maybe @willnationsdev can give it a check? It touches on some stuff related to his work.

[YuriSizov]

Code looks good, and the mentioned features work.

One minor problem I've noticed is that the icon for the node, when creating a scene from a custom type, is not displayed correctly initially, but fixes itself after the scene is reloaded.

[akien-mga]

Thanks!