Add ability to programatically remove media_source's from media items.

[kenmcgaugh]

This adds a remove_media_source_atom and associated message handler in the media_actor to allow for removing media sources. Note that removing all media sources from a media item puts xSTUDIO into a bad state and so should be avoided. We implemented a version changing system like the following, acting on a media item:

1. Get the current MT_IMAGE media_source.
2. Set the media_reference URI of this media_source to point to a new version of the media.
3. Use the above remove_media_source_atom message to remove all sources except the current one.
4. Send message to session to trigger the gathering of additional sources based off of the new current source.