Review the inability to create scenes with reserved names

Seeing as 'startup.txt' and 'choicescript_stats.txt' are compulsory scene files in a ChoiceScript game, CSIDE is quite strict with their handling (to prevent newbie authors making silly and frustrating mistakes like deleting or overriding them).

Whilst some of these decisions I stand-by (e.g. preventing deletion inside the application), some don't make sense, for example:

Which was recently re-highlighted by Vendetta.

The only time you'd want to rename a scene to startup is if you didn't already have a startup scene. If you did have a startup already, then the rename will fail anyway:

So this check probably doesn't have much value, and is prone to causing frustrations of its own, as it makes it impossible to re-create these files from within the application (outside of creating a brand new project).