[WIP] Ability to serialize state

[captbaritone]

Just the very first groundwork for #654.

My thoughts so far:

1. Rather than serializing the Redux sate as is, we should explicitly add the stuff we want to serialize and handle serializing/deserializing it manually.
2. This will be so much easier once we have Typescript.
3. We must be sure we can handle old serialized values.
4. Users should construct a new Webamp instance and then manually restore state as a second step, rather than being able to construct a new instance from a serialized state.

[captbaritone]

Probably worth waiting for Typescript support: #657.

[captbaritone]

Replaced by #658