Support areas with more than one building map (shop, ranch, etc)

[Ruin0x11]

This would mean migrating what constitutes a shop from the area to each individual map. This shouldn't be too hard, because all the code does is take the area and load floor 1. We'd just have a list of individual map UIDs tied to buildings instead, and remove them when its containing area gets removed.

[Ruin0x11]

Depends on #179.

[Ruin0x11]

I don't know why we'd work on this now; it's a pretty esoteric use case. If someone wants to have this in the future, I'll re-open this issue.