Replace gnome-screensaver with xscreensaver

[Lars Stokholm]

Since I noticed that gnome-screensaver (at least with the current
port) hasn't got options for monitor power saving (as xscreensaver
does), I want to replace gnome-screensaver with xscreensaver, as I'm
offered to, during the compilation/installation of the GNOME port. How
would I do that? Force a deinstallation of gnome-screensaver and
install xscreensaver afterwards?

I'd also expect xscreensaver to integrate into the Desktop >
Preferences menu - is that too optimistic?