window-list: Fix primary button action on touch

If a click event was triggered via touch rather than a pointer
device, the button parameter is 0 rather than a mouse button
number.

Account for that to make sure that touch events are not misinterpreted
as right clicks.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/146

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/233>