refactor: Significantly refactor how conditions work internally (#1886)

- Much improved API cleanliness and testability to allow further
extensibility in future
 - Simplified code in `live` view

This technically contains a small change in how overrides work in the
`live` view. Since that change is _closer_ to the documentation, and
since this is likely to be rarely used, this is not considered a
breaking change. Previously, overrides for a given live camera would
always render _as if_ that camera was selected, vs was actually
selected. Now, overrides will only apply in the live view when the
camera is _actually_ selected. If this is an issue for you in practice,
lets discuss.

src/conditions/state-manager.ts

@@ -0,0 +1,60 @@
+import { isEqual } from 'lodash-es';
+import {
+  ConditionState,
+  ConditionStateChange,
+  ConditionStateListener,
+  ConditionStateManagerReadonlyInterface,
+} from './types';
+
+/**
+ * A class to manage state used in the evaluation of conditions.
+ */
+export class ConditionStateManager implements ConditionStateManagerReadonlyInterface {
+  protected _listeners: ConditionStateListener[] = [];
+  protected _state: ConditionState = {};
+
+  public addListener(listener: ConditionStateListener): void {
+    this._listeners.push(listener);
+  }
+
+  public removeListener(listener: ConditionStateListener): void {
+    this._listeners = this._listeners.filter((l) => l !== listener);
+  }
+
+  public getState(): ConditionState {
+    return this._state;
+  }
+
+  public setState(state: ConditionState): void {
+    this._processStateChange(this._calculateTrueChange(state));
+  }
+
+  protected _processStateChange(changeState: ConditionState): void {
+    if (!Object.keys(changeState).length) {
+      return;
+    }
+
+    const oldState = this._state;
+    this._state = {
+      ...oldState,
+      ...changeState,
+    };
+    this._callListeners({ old: oldState, change: changeState, new: this._state });
+  }
+
+  protected _calculateTrueChange(change: ConditionState): ConditionState {
+    const changeState: ConditionState = {};
+
+    for (const key of Object.keys(change)) {
+      if (!isEqual(change[key], this._state[key])) {
+        changeState[key] = change[key];
+      }
+    }
+
+    return changeState;
+  }
+
+  protected _callListeners = (stateChange: ConditionStateChange): void => {
+    this._listeners.forEach((listener) => listener(stateChange));
+  };
+}