Recording actions about calibre

[jeremytregunna]

I'm not sure that the app state has to be saved along with the actions, I think we can compute that when an action is replayed. This has some benefits:

• We don't need to track this state on the filesystem
• Users don't have to conform to any given protocol on their StateType objects

but it also contains some disadvantages

• In much the same way you iterate a singly linked list, you have to start from the beginning, and apply actions until you get up to the one you want.
• Actions must conform to a protocol that allows the action to be convertible into an object we can serialize

[jeremytregunna]

Currently I'm thinking of providing a protocol for encoding/decoding actions that leaves the actual job of decoding/encoding up to the implementor. When recording, you'd pass in an instance of this thing which the store would then own, and provide a default action implementation of a representation property which builds up a dictionary of String->Any which is passed into the encode method, returning an NSData, and decode would take an NSData and return an instance of the action