Winops layer menu: Add "Lowest"

Also rename "On Top" to "Highest".

config/strings.c

@@ -145,10 +145,11 @@ const char         *txt[] = {
    _("Destroy a group this window belongs to"),
 /* winops_layer.menu */
    _("Stacking"),
+   _("Lowest"),
    _("Below"),
    _("Normal"),
    _("Above"),
-   _("On Top"),
+   _("Highest"),
 /* winops_misc.menu */
    _("Miscellaneous"),
    _("Toggle Skip Window Lists"),

config/winops_layer.menu

@@ -1,5 +1,6 @@
 "Stacking" "EMPTY"
+"Lowest"	NULL "wop * la 1"
 "Below"		NULL "wop * la 2"
 "Normal"	NULL "wop * la 4"
 "Above"		NULL "wop * la 6"
-"On Top"	NULL "wop * la 8"
+"Highest"	NULL "wop * la 8"