Rather than having the mapping from window into "slots" (or
possible positions in the workspaces) be dependent on stacking
order, compute the minimum-motion which is a vector from one
top left corner to another. This order won't change as long
as the window set and their positions stay fixed.
There are two minimum motion algorithims; one simply computes
all possible placements by permuting the window list, up to
a current maximum of 5 windows. Past that (which also happens
to be the number where we switch to a grid), we use a "greedy"
algorithm which for each slot, finds the window with least motion
for that slot.
To break any ties, we use an internal integer in MetaWindow which
enumerates the order in which windows were created.
https://bugzilla.gnome.org/show_bug.cgi?id=582654
7703bee284