Recognise the desktop window as uninteresting.

In Sawfish, when choosing whether to minimise the window under the
pointer or the one with input focus, we should consider windows which
test true under desktop-window-p to be as uninteresting as 'nil' - if
the pointer is on that, it's _basically_ the root window, and should
be treated as equivalent for the purposes of falling back to something
else.


git-svn-id: svn://svn.tartarus.org/sgt@10192 cda61777-01e9-0310-a592-d414129be87e