Just as we did for the workspace switcher popup, support workspaces
being laid out in a single row in the window picker.
Note that this takes care of the various workspace switch actions in
the overview (scrolling, panning, touch(pad) gestures) as well as the
switch animation, but not of the overview's workspace switcher component.
There are currently no plans to support other layouts there, as the
component is inherently vertical (in fact, it was the whole reason for
switching the layout in the first place).
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/575
305e63750e