Extend media renditions (including additional actions) in Glib API

In order to make a viewer that properly supports embedding media using the Glib interface, a couple of things are missing.

• access to additional actions on annotations.

  This will allow in particular auto play/pause/stop etc.

• being able for a given rendition action (either from additional actions or triggered by a clicked annotation, etc.) to get the information:

  1. Which screen annotation is this action targetting?
  2. Which media is the action specifying?

  This would allow to know where to render the rendition-speicified media, and whether it should replace the previous media displayed in that annotation, or whether it should just modify the state of the currently displayed media.