Only fire setting changes for changed settings

Previously we were firing updates for everything, which is bad. This has an effect of causing the room list to update itself every time the user goes to toggle some account settings.

src/settings/handlers/MatrixClientBackedSettingsHandler.js

@@ -42,6 +42,10 @@ export default class MatrixClientBackedSettingsHandler extends SettingsHandler {
         MatrixClientBackedSettingsHandler._instances.push(this);
     }
 
+    get client() {
+        return MatrixClientBackedSettingsHandler._matrixClient;
+    }
+
     initMatrixClient() {
         console.warn("initMatrixClient not overridden");
     }