core: Refactor two new methods of ContentDirectory
core: Refactor two new methods of ContentDirectory

Refactor get_transfer_progress_cb & stop_transfer_resource_cb methods of
ContentDirectory.

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