Permit selling houses; reform company sale calculation

[lindleyw]

• Attempting to purchase a house already owned, will sell it back (it will again be "unowned") and the purchase price will be refunded. This opens the eventual possibility that if the house or land value has changed, the value recouped may be above or below the original purchase price.
• Houses are still not tracked as part of a company's assets, which will have to be done for the "Assets" figure on the financial report to be correct, or for completely proper company takeover prices. Perhaps a future pull request might do this.
• Reform asset and liability computation for company sale, as being the higher of known assets or known liabilities. Liabilities include original starting cash, plus any short-term loans (negative cash account balances).

[dannyman]

This is potentially very neat, as it allows for right-of-way land speculation. (Can we buy unimproved land?)

[jamespetts]

Interesting, thank you for this. Can I check what the UI is for selling a house - do we need a new graphic for a button somewhere?