Distinct icon for Secret Door warning

[AdamPlenty]

Currently, the warning for a Secret Door being spotted uses the generic purple ! icon, and I've noticed that it often gets overlooked as a result, because the icon is often used for far more trivial warnings. Since it is pretty serious, it ought to be made more distinct.

[benlp91]

@Loobinex can be closed ? you don't like the idea you said