Create a single object to receive the deltas of workflow-subscriptions

[kinow]

Describe exactly what you would like to see in an upcoming release

At the moment we have the CylcTree object which works well for the Tree View. But we want to have another structure, that is created before the CylcTree.

This structure will be shared by multiple workflow views, like Table view, Tree view, and Graph view.

It will receive any deltas from a workflow-subscription (ideally merged sub of all the views, to be discussed elsewhere). Once a delta is applied to this structure, it must notify any listener/observer via Vue reactivity.

Additional context

The CylcTree uses a flat lookup map (JS Map object). This object could be our flat object, passed around to other views, that can filter it, or derive new structure like CylcTree, or some array for the Table View.

One premise that must be followed, is to avoid duplication. So if a delta arrives for a subscription, we want to get that object from the delta, and store it only once. We can have pointers/references to this object. But we must not duplicate it.

This object should work with the current Tree view, and with the new Table view in #672

Pull requests welcome!