The code assumed that the focus window was always the one at the
top of the window stack, which is not true if an unfocused window
has the above hint set.

Rather than fixing this assumption, rename the function to
lower_beneath_grab_window() and use the display's grab window - the
function is only used for displaying the tile previews, which means
that we want the grab window anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=650661