Aircraft leased from other player: Enable buy

[JumboShrimp]

When a player leases aircraft from another player, and aircraft is on the market as buy or lease, enable buy option for lesee.

For lessor, allow more control on aircraft leased leased out: enable changing buy / not buy, enable setting max contract renewal time.

[vitongwangki]

I guess this would help subsidiary airlines to build better, because they need not return his plane to lessor when they want to buy that particular plane.

And I guess the default option of "buying by customer" should be "Not allowed to buy". It is because the buying action is complicated, and the lessor may not always want to sell the plane when they secured the leasing deal. And the price of buying/selling should be able to set at anytime by the lessor, not according to the initial leasing price.