screen: Make sure to initialize NUMBER_OF_DESKTOPS

update_num_workspaces() is a no-op when the number of workspaces
did not actually change. That is fine, except that we still want
to initialize the _NET_NUMBER_OF_DESKTOPS hint on startup to not
break components like nautilus-desktop that rely on it.

https://bugzilla.gnome.org/show_bug.cgi?id=760651