core: Refactor default icon addition code
core: Refactor default icon addition code

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