[Suggestion] Implement "A secret is revealed!" message from PrBoom+

[ShadowOne333]

It was recently brought up to my attention that PrBoom by itself didn't have the option to show the "A secret is revealed!" message upon stepping into a secret, which follows the same behaviour as vanilla Doom.
I'd like to suggest adding said feature of showing up the message upon finding a secret, perhaps backporting it from PrBoom+, to allow for such an option into PrBoom by itself.

PS: Side question, does the PrBoom core have any option to set the aspect ratio to 16:9?
Having the aspect ratio as "core-provided" makes it 4:3 always.