Can't rename scene to a reserved scene name even if they don't exist

CSIDE tries to prevent you from overwriting/deleting important scenes (startup, choicescript_stats), in order to keep a project functional. However, this can be problematic if the user is importing a new project and wishes to create a startup/choicescript_stats scene for it from within CSIDE.

I imagine this isn't going to be very common (you'd typically either create something new in CSIDE, or import something that has the important scenes already), but it's reasonable to expect CSIDE to be able to handle this case nonetheless.