Implement action-based global state management

Current state-management is non-reactive code-pasta.

**Solution:**
Make the state reactive. Implement an observable state reducer with action-dispatch based state mutations.

**Scheme (a):**
Tools:
RxPY (https://github.com/ReactiveX/RxPY)

Coupling:
Observable pattern. Components subscribe to global state (RxPY Observable/Subject) and receive a dispatch function.

**Scheme (b):**
Tools:
Python Class for managing global state.

Coupling:
Observable pattern. Components subscribe to global state and receive a dispatch callback from the parent.

The issue names no files or tests. Start by locating the current global state-management code and the components that consume it, then compare the RxPY and Python-class schemes against the existing architecture. Done means state changes are observable and mutations go through dispatched actions, with affected components receiving updates.

Written by the indexing model from the issue text.