Change DonateAction to extend TransferAction (instead of TradeAction)

[trentlarson]

All the other TradeActions involve a swap of items or an intention of each party both giving something and receiving something in return. (In fact, in broader contexts, there's a phrase that "trade is the origin of most problems", and the concept is distinguished from gifts and donations.) TipAction is another possible difference, but it is done in return for good service... I could see an argument for it either way.

DonateAction seems distinctly different, explicitly "without compensation", so I propose to make it a subclass of TransferAction instead.

[trentlarson]

There are 3 properties of TradeAction that would be lost: price, priceCurrency, and priceSpecification. One solution for pricing is to make the object be a MonetaryValue or TypeAndQuantityNode -- and if there's already an object then that might be turned into an array (and do we really need priceSpecification?).

[trentlarson]

I've made this at schema.org: schemaorg#3338