Add support for showing named place on map

This shows “named place on map” (P9664) statements on the image just
like “depicts” (P180) ones, but in a different color (fuchsia: a color
hopefully not common on maps – unlike, say, green or blue). Existing
statements can also have their regions added or edited. However, it is
not yet possible to add new P9664 statements in the tool.

I originally hoped to also show “inscription” (P1684) statements in a
very similar way, but those statements don’t have item IDs as values, so
that will require some more work.

Part of #2.