server,media-export: Fix media-art extraction

- mime is meant for the buffer contents
- Do a proper fall-back to sidecar search

Signed-off-by: Jens Georg <mail@jensge.org>
server: Port to new mediaart API

Signed-off-by: Jens Georg <mail@jensge.org>
media-export: Work-around bgo#739915

Signed-off-by: Jens Georg <mail@jensge.org>

Conflicts:
	src/plugins/media-export/rygel-media-export-item-factory.vala