Layout.Spacing applies a customizable amount of space around the outside of each
window. At window edges where two windows meet, the total distance between them
is therefore twice the customized value (one space value from each window). At
the edge of the screen, however, the spacing is only applied once. This results
in uneven amounts of spacing and differently-sized gaps on the screen.
This patch extends the Spacing layout to include a further gap all around the
edge of the screen, thus making all spaces around windows equal in size.
