I was doing nasty things with a for loop - we've replaced it with a
map statement, which achieves the same thing but works better with
Vue.js reactivity.
Each ImJoy window now gets its own top-level tab.
This involved some fairly significant refactoring of the ImJoy
component, but I think it's cleaner now.
I've moved imjoy-related stuff into a store "module" to keep it clean.
Specifying overrideOrigin in the URL now
sets up the store with the modified origin.
This is intended for development, and makes it
much less annoying to develop with a Vue dev server.
