workspacesView: Support horizontal layout

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).

GNOME/gnome-shell!575
4 jobs for !575 with classic-ws-layout in 4 minutes and 52 seconds
detached
Status Job ID Name Coverage
  Review
passed #328937
check_commit_log

00:00:56

 
  Build
passed #328938
build

00:02:49

 
  Test
passed #328939
test

00:01:05

passed #328940
test-pot

00:00:55