Add notification actions: Next, Custom commands, and Open details

This PR focuses on adding notification actions to the existing new artwork notifications in an effort to make it easier to interact with Muzei (being particularly important on Android Wear devices).

For sources that support the 'Next Artwork' built in action (`BUILTIN_COMMAND_ID_NEXT_ARTWORK`), we now show a 'Next' action in the notification so that users can skip to the next artwork in response to a notification.

Only for Android Wear devices, we also include a 'More' action which brings up the list of custom user commands so that the user can take these custom actions without pulling their phone out. This only appears if the source registers custom user commands.

For sources that include a `viewIntent` in their artwork, we now show a 'Details' action which directly links to this intent.