Consider switching from Vuex to SBP-based store

[taoeffect]

Problem

Although Vuex is very nice and has great dev UI support via the vuejs browser extension, it ties us to Vue and makes switching to something like Svelte (#587) (or any other framework that comes along) more difficult.

Also, Vuex does represent another dependency that we can potentially get rid of (#372).

Solution

Create an SBP-based alternative to Vuex, in a way that's compatible with Svelte (and ideally arbitrary frameworks).