[Greasemonkey] Bait and switch script installation

[Anthony Lieuallen]

> Good catch. Why is GM downloading a new copy, anyway?

I agree.  The current system looks really silly.  DM pop up, usually
disappear almost instantly, sometimes hang around for a while.  DM
shouldn't show at all, because I use download statusbar, but it does. 
Worse, I get an entry in download statusbar too!  Would be very nice
to simply use the contents of the window.