Don't show project settings starting with underscore

[KoBeWi]

If you add a project setting that begins with underscore (e.g. _some_setting, without slashes), it will display under Global category. It's inconsistent with editor settings, which don't show underscored settings. This PR fixes the inconsistency.

[AThousandShips]

Makes sense

[Calinou]

It might be worth documenting this fact in the ProjectSettings and EditorSettings class reference, as editor plugins may want to rely on this in specific scenarios.

[KoBeWi]

ProjectSettings have set_as_internal() for hiding properties. This is more relevant when editing project settings without a plugin (which is how I discovered this bug).
EditorSettings have no equivalent method however.

[akien-mga]

Thanks!