Supply do_pull and do_seek_on_read to cmis backend
do_pull is used by a number of GVFS users to retrieve entire files
from the backend. This function allows programs like evince and gedit
to work correctly.
do_seek_on_read allows applications to seek while reading. I had hoped
that adding this function would allow gimp to open files over this backend
but it did not seem to work. (PDF file tested) FWIW, gimp couldn't open
the files over webdav either.