Allow ruleset to allow dismantling (selling) some wonders

[Corbeau-1]

Reason: this way the wonder mechanic can be used with more flexibility and for more technical purposes. Wonders can give benefits and penalties at the same time and the player can balance what he needs in what time period.

How to distinguish sellable from non-sellable wonders:

Quick Dirty Patch: If the cost of the wonder ends with a 5 (15, 25, 105, 1005...) the wonder can be sold. If it's a round 10, it can't. This may lack some elegance, but it would only require a ruleset maker to pay attention to this.

Elegant Solution: Create a flag "Sellable". If the wonder has the flag, it can be sold. If it doesn't, it can't.