gsequence: Improve is_end()
gsequence: Improve is_end()

Instead of finding the GSequence, just walk up
the tree and determine if the iter is the end node.

https://bugzilla.gnome.org/show_bug.cgi?id=749583
Signed-off-by: Garrett Regier <ad31fbc9887f4905727595ab366ed8d35c507682@gmail.com>

johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib