[WIP] Switch from onPlayBack* callbacks to onNotification as it's more reliable.

Also document all the player "types". Old code already emitted events with the
correct type, but the documentation was wrong.
Search for "Notifications.Item.Type" in
https://github.com/xbmc/xbmc/blob/master/xbmc/interfaces/json-rpc/schema/types.json
for all supported types.