Implement support for watching for changes in settings

This implements a dream of one day being able to listen for changes in a settings to react to them, regardless of which device actually changed the setting. The use case for this kind of thing is extremely limited, but when it is needed it should be more than powerful enough.

src/settings/handlers/ConfigSettingsHandler.js

@@ -47,4 +47,12 @@ export default class ConfigSettingsHandler extends SettingsHandler {
     isSupported() {
         return true; // SdkConfig is always there
     }
+
+    watchSetting(settingName, roomId, cb) {
+        // no-op: no changes possible
+    }
+
+    unwatchSetting(cb) {
+        // no-op: no changes possible
+    }
 }