Restrict Autoloads from having keywords as their names

[YeldhamDev]

A re-take on #22107, fixes #17494.

A comment in the original PR suggests using ScriptServer::is_scripting_enabled(), but as it always returns false, this condition is not used.

Some small enhancements where also added:

• Passing an invalid name now only clears the name field, avoiding having to choose the path again.
• Pressing Enter while having the name field focused now counts as having pressed "Add".

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While doing this, I also made an interesting discovery: Classes registered which names start with "_" (such as _Engine, _OS, etc), even though they can be identified without the underscore while scripting, searching for it in the ClassDB will result in nothing without it.

This explains why you can name Autoloads which such classes, and also why they don't inherent the Object icon in the new Editor Help dialog.

Now here's the question: Should we make those two places also check the class names with "_" when searching the database, or should something be done in the ClassDB itself?

UPDATE: The above can be fixed by either #26990 or #26922.

[akien-mga]

Thanks!