[plugins/spotify] fix broken integration

Fixes Spotify integration.

In the future, I recommend not using purely hierarchical css/xpath selectors as they will be very brittle, breaking on any minor change to the DOM tree.

Additionally, it's better to query/test for attributes/classnames that aren't meant to contain text to be displayed to the user (e.g. don't use titles as previously used in getPlaybackStatus), because it's likely that Spotify's i18n will cause breakage.