Multiscreen fixes. Attempt to fix at least part of bug #2693.

plugins/actions/actions.c

plugins/clock/clock.c

plugins/iconbox/iconbox.c

plugins/pager/pager.c

plugins/separator/separator.c

plugins/showdesktop/showdesktop.c

plugins/systray/systray.c

plugins/tasklist/tasklist.c

plugins/windowlist/windowlist-dialog.c