Add next artwork and open details notification actions

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.

For sources that include a viewIntent in their artwork, we now show a 'Details' action which directly links to this intent.
Show custom actions as selectable list on Android Wear devices

To give users full access to any custom actions available from sources, we provide a 'More' action on Android Wear which brings up the list of custom user commands so that the user can take action directly from their Android Wear device
Adds 'More' custom action icon for Android Wear devices
Ensure Wear actions are marked as not available offline (when disconnected)

Actions that trigger PendingIntents on the Muzei app (all three at this point), should be marked as unavailable when the Wear device is unattached.
Merge pull request #89 from ianhanniballake/notification_actions

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