overviewControls: Adjust heuristic for hiding workspace switcher

overviewControls: Adjust heuristic for hiding workspace switcher

The workspace switcher should be expanded when workspaces are in use.
Our current implementation assumes that workspaces are used when there
are windows on at least two workspaces. However workspaces are already
used when moving from a non-empty workspace to an empty one (presumably
with the intention to launch something on that workspace), so tweak the
heuristic accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=662457