Move multi-file editor store to Vuex

What does this MR do?

Moves the multi-file editor store into Vuex. This makes managing the state more manageable & understandable as it all now happens in the one place. Also with this a lot of code was removed that either wasn't doing anything or wasn't very performant.

With the store changes we get a small boost in performance with Vue because each component is only watching what it requires whereas previously the store was added to all components. Previously the store had a lot of extra data inside of it that wasn't required, some of these have been moved to components others have just been removed.

With this new store it will also help us extend the multi-file editor in the future because it makes the data management much easier to understand. We could plugin different services for example & still keep all the same data structure.

Also whilst I was doing this I have updated a lot of components to match our docs.