Infrastructure for reactive UI

[CodeWringer]

Traditionally, in order for UI to react to state changes, UI updates have to be triggered imperatively, from wherever the business model state has been altered. Although easy to implement, such a UI infrastructure is difficult to maintain, because one has to consider every current UI element that may need to be touched when any particular piece of data changes.

So instead, a more modern approach should be supported: UI elements listen for changes to the data related to them. Whenever that data is changed in any way, the UI elements interested in that data update themselves. This is a paradigm-shift from imperative to (mostly) declarative UI programming, which is far easier to maintain in the long run, but more work up-front to set up.

With this issue, the up-front work is to be completed. Its requirements are as follows.

1. Data changes must be observable by UI elements interested in them.
2. When a UI element is staged, it should automatically subscribe to the data it's interested in.
3. When a UI element is unstaged, it should automatically unsubscribe any and all data changes, to obviate lingering references.
4. How the data changed must be trackable. E. g. when an element in a collection is added, removed, moved or edited, then this specificity must be passed on to listeners. The same goes for non-collections. When a value is changed or any value within its nested properties is changed, then the exact change must be passed on to listeners.