This is the most basic version of a workspace switcher, taken from
Frippery Bottom Panel and adapted. It handles clicks and scrolls,
and does not show window thumbnails or shapes.
Note that, differently from the frippery version, it won't change
the workspace layout, and actually assume a linear vertical layout,
which is then shown horizontally. This is to keep compatibility
with the overview, which uses a vertical layout.
https://bugzilla.gnome.org/show_bug.cgi?id=694914
f86f09955b