core: Slightly correct previous commit 487816
core: Slightly correct previous commit 487816

 - ImageItem implies !streamable
 - MusicItem / VideoItem imply streamable

Didn't see that during review.

GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,luisbg/rygel,Tarnyko/rygel,GNOME/rygel,jku/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,GNOME/rygel