The thumbnail controls are not a separate actor in the overview group
yet, so we need to ensure a spacing between them and the workspaces
view.
Instead of exporting the overview spacing, just add a temporary style
class to the workspaces-view actor for it. It will be removed in the
future when we change the layout of overview elements.
https://bugzilla.gnome.org/show_bug.cgi?id=690174