[WIP] Signs

Game flavor. Good for marking dangerous areas (mine fields, see issue #7800) or other things.

- [X] Sign construction and furniture def.
- [ ] Writing on sign (add a item_vars member to furniture like item class?).
- [ ] examine function for signs (reading and writing new text on a sign).
- [ ] sign writing shows up in info when cursor is over the sign.
- [ ] spotting a sign from the distance.
- [ ] auto marking a sign on a map if auto notes is enabled.

Open to suggestions, the above are what I'd like to put in.