Fix wibox.layout.stack drawing the top layer at the bottom

The top layer was being drawn under the other layers. Fixed by iterating
the widgets table backwards, so that the top (index 1) is drawn last.

lib/wibox/layout/stack.lua

@@ -74,7 +74,9 @@ function stack:layout(_, width, height)
 
     local h_off, v_off = spacing, spacing
 
-    for _, v in pairs(self._private.widgets) do
+    -- Iterate backwards to draw top (index 1) last so that it appears above other layers
+    for i = #self._private.widgets, 1, -1 do
+        local v = self._private.widgets[i]
         table.insert(result, base.place_widget_at(v, h_off, v_off, width, height))
         h_off, v_off = h_off + self._private.h_offset, v_off + self._private.v_offset
         if self._private.top_only then break end