refactor how we keep track of entities

[bahrmichael]

For #321

We currently have a couple different ways to keep track of entities. This has evolved over time, and with #321 we may now have a good reason to refactor that. For example the town hall is stored as a field on the city, and other supply buildings are stored on a separate field which is invalidated and repopulated occasionally. And treasuries? No idea.

I'll document the current approaches here in more detail, and will then come up with a unified approach.

Todo: document calls to surface.create_entity, that should be easier than reading all the code

[winex]

i'd like to point to such calls as well as i think it fits here

factorio-tycoon/construction-event-handler.lua

Lines 34 to 40 in fa921c7

	-- TODO: this could be dropped in favor of Util.getGlobalBuilding()
	if global.tycoon_entity_meta_info == nil then
	global.tycoon_entity_meta_info = {}
	end
	global.tycoon_entity_meta_info[entity.unit_number] = {
	cityId = city.id
	}