Prototype purchase history

[cassidyjames]

So AppCenter will show an "app history" or library of some sort (elementary/appcenter#1229) and support reinstallation, but the wallet should show a sort of transaction log of all purchases. In the future, this could support showing non-app-related purchases, i.e. other content, OS payments, etc. as well. Because of this, I don't think it needs to support reinstalling apps from here or anything, but I'm open to discussion.

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