Make sure user can't add double item to playlist

[robbevp]

Fixes #819

This playlist fixes two things around double items in playlists:

• The error returned by the api (see Return error when creating double playlist items api#352) is now translated and correctly displayed
• The options that already contain the item are disabled. I've also added a message to explain the disabled state (This message is quite verbose, but I don't mind this)

Since the error should not occur that often anymore, I don't think it's a problem that it is shown in its usual place and not in the dialog.