Are manners all about social signaling? I don't think so. I find bad manners to be an aversive stimulus, and poor table manners cause me to lose my appetite. To term that a "signal" is to stretch the definition. Once a society agrees on manners, unmannerly behavior becomes offensive.
It's not rocket science. The basics, for kids:
Take your elbows off the table.
Don’t talk with your mouth full.
Don't pick up a fork until everyone is seated and the hostess picks up her fork.
Eat slowly, and converse during meals.
Look people in the eye when you speak to them.
Avoid talking about yourself: Assume that only relatives and close friends care.
Say "Please" and "Thank you," and say "How do you do?" when introduced to elders.
Give a firm handshake.
Write your thank-you notes in a timely fashion.