core: Always time-seek if not byte-seeking
core: Always time-seek if not byte-seeking

Always time-seek if duration is known and byte-seek is not feasable.

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