workspace: Center-align window previews if there's only a single row
Instead of always aligning window previews vertically at the bottom of
their row, only do that if we have multiple rows. If there's only a
single row of windows, align every window vertically centered.
This is a very small step towards the new layout for window previews in
the overview, but since the release of 40 is getting nearer and nearer,
changing more is not feasible anymore.