City growth

[Norgg]

We should start the city out small and grow it. We can probably have the main roads already spawned, but smaller roads and buildings off them should then develop as the city grows. The first few buildings should be smaller ones that then get demolished and replaced with larger ones as the population grows.

[Riot]

populating lower levels of buildings on plots at city generation, re #22

→ <<cset 2c5f52136b36>>

[Riot]

adding debug mode context menu buttons for upgrading and downgrading buildings, re #22

→ <<cset 0491c01852d8>>

[Riot]

don't add duplicate buildings of the same type to allowed plot types, re #22

→ <<cset 3bd2175f7047>>