I'm trying to add a slider to an indicator to port my gnome-shell extension to unity. I found out this question which is exactly my problem. If this is not possible using libido and indicator how is the sound indicator implemented ?
The sound indicator is not exported from an application. If you create a media-player, you never specify that there are sliders or play-pause buttons, you just say, that the application is a media-player.
Your problem is, that you can not sent the information "I need a slider" to the software which creates the menus. It is not that this software can not create sliders.
For details of the implementation, you can look at the code: