Change the layout strategy to be more like the mockups. With less than
two rows of windows, we try to fit every window in a non-aligned situation;
with more than three rows of windows, we try to fit every window in an
aligned situation.
Based heavily on a patch from Pierre-Eric Pelloux-Prayer <pelloux@gmail.com>
https://bugzilla.gnome.org/show_bug.cgi?id=582650