window: Avoid spurious focus-window changes when showing desktop

When using the show-desktop shortcut with no desktop window, unshowing
will focus the second-most-recently-used window. If we find a desktop
window, it will be focused explicitly and everything works as expected;
however without a desktop window, we end up hiding the focus window,
which will use focus_default_window() with the not_this_one parameter
to move focus away. We used to get away with this, as the not_this_one
parameter was ignored until commit e257580b, now with bug 675982
fixed, we need to explicitly handle the show-desktop case.

https://bugzilla.gnome.org/show_bug.cgi?id=686928