optical-media: Handle cancelled retvals like errors

https://bugzilla.gnome.org/show_bug.cgi?id=722629
optical-media: Track added media

https://bugzilla.gnome.org/show_bug.cgi?id=722629
optical-media: Add a cancellable for other ops

https://bugzilla.gnome.org/show_bug.cgi?id=722629
optical-media: Don't pass browse data to entry_parsed_cb()

https://bugzilla.gnome.org/show_bug.cgi?id=722629
optical-media: Add support for content-changed

And rework the plugin to handle and monitor mounts rather than
volumes. Even though we'd like to support volumes, it's rather
unnecessary and causes problems:
- physical discs will get mounted anyway, so create a mount
- ISO images, when added, generate volume events that aren't
  easy to process when we're interested in mounts ("added" without a
  mount, "changed" when the mount is added)
- When removing ISO images, the backing loop device might not get
  disconnected, not generating volume events, and when we get mount
  events, it's too late to find the backing volume

All this together means that the the code is barely readable after
all those changes, so here's a chunk of it.

https://bugzilla.gnome.org/show_bug.cgi?id=722629
optical-media: Add start/stop notification changes

https://bugzilla.gnome.org/show_bug.cgi?id=722629