Add ability to sort change list

Added key bindings to sort the change list by number (current default), and by
the time the change was last updated. Also added the ability to reverse the
sort, useful for showing the most recent changes at the top of the screen.

Doesn't depend on but sorting by updated column makes more sense with this
patchset: https://review.openstack.org/#/c/122176/

Change-Id: I4d0ee462c106e26857785c9b4e26cc17e906796a
Merge "Add ability to sort change list"