PC birthday event

This MR adds a birthday event for the PC. It includes a few scenes with some basic variants. These scenes are pretty damned vanilla for what's possible in FC, which I justify because:

• this MR is kinda of testing the waters, and it seemed like starting with broader appeal is better than narrower;
• I see this as kind of a starting point, from which the event can be (and probably will/should be) extended, improved, and customized; and
• any further changes could use with some guidance, anyway.

Also I've sort of decided that the combinatorics possible now make writing certain types of scenes nearly infeasible. For example, I don't check that the party planner has arms and legs, or both eyes, etc., even though that could be added relatively simply. But also I've sat on this long enough that I'd rather get feedback before doing much more.