Implement '<city> can't support <building>' popups when treasury is empty.

[iStruggle]

Not sure on the order of selling though but building should be autosold and funds added to treasury.

I also remember seeing an advisor window saying funds were getting low and I should consider adjusting the rate or employing taxmen, but I didn't get that when testing though on the original.

[SWY1985]

Linked to #144

[iStruggle]

sorry, didn't spot that you'd already got this.

[SWY1985]

No problem, better to actually have a seperate issue about this.

[SWY1985]

I can't figure out how Civilization decides which building to sell. I'm going to move this issue to the next version for now, unless we suddenly figure out how the original game does this.