[Controller] Disallow multiple TildaWindows to bind to the same key

[Controller] Disallow multiple TildaWindows to bind to the same key

Previously, it was possible to have two TildaWindows be simultaneously pulled
down via the same key binding. This has the potential to be very cool, but also
very confusing. For now, completely disable this ability. In the future, it
will be re-enabled via a hidden option.