awful.placement.no_overlap caused clients on an otherwise empty desktop to stich
to the top-left corner of the screen because it always preferred the top-left
corner of the available space.
This commit doesn't change the way in which the available area is computed, but
instead this now tries to keep the client at its original position if that is
available.
This fixes cases where calling awful.placement.no_overlap as a callback from an
awful.rule rule didn't work because the manage signal in the default config
later overrode that via calling awful.placement.no_overlap.
Signed-off-by: Uli Schlachter <psychon@znc.in>