Made hidden ProjectSettings groups more explicit

[KoBeWi]

Follow-up to #61751
Changes long if into a more clean list of prefixes.

I experimented with autoloads. At first I tried marking them as internal when added, but the problem is that this isn't saved. The properties are simply loaded with ProjectSettings resource and then re-flagged. So there is no avoiding _get_property_list() or _validate_property() without some bigger changes that just aren't worth it.

[KoBeWi]

Rebased. I moved the prefix definition to their respective part of code. I also changed the prefix container from List to LocalVector, which I think is faster.

[Calinou]

Code looks good to me.

Note that this doesn't change the editor UI in any way, even when Advanced Settings is enabled.

[AThousandShips]

It appears to add a lot of extra data to the docs, something broken in the implementation?

[KoBeWi]

Fixed, I had to move input/ prefix back to ProjectSettings.

This is a funny problem. InputMap is created before ProjectSettings, so add_hidden_prefix() can't be used in its constructor. The 2 earliest methods InputMap are load_default(), which is called once in Main when the editor starts, and load_from_project_settings(), which is also called in Main, but is also a bound method, so adding prefix there would cause problems. Apparently doctool runs in non-editor context, hence calling the prefix method in load_default() did not work correctly. I think the best fix would be creating InputMap after ProjectSettings, but it's risky, so I just made an exception.

[akien-mga]

Looks much cleaner.

[akien-mga]

Thanks!