Structure context menu

Okay here are the basics for a context menu that appears when you click on a building. Right now it features a slider that you can drag to allocate citizens. The capacity of citizens that a building can have is configurable. At some point  I will implement a restriction on the allocation that checks if you actually *have* enough citizens. Also, I changed that right after placing a building you're out of placement mode. This way you can actually assign citizens straight away. We'll have to playtest to know how this works, maybe it's disturbing the workflow of placing a whole bunch of buildings.. dunno