endSessionDialog: Simplify CSS padding handling

... so that we add more items to messageLayout without having to
hardcode the same padding in each of the children.

https://bugzilla.gnome.org/show_bug.cgi?id=722898

js/ui/endSessionDialog.js

@@ -240,7 +240,8 @@ const EndSessionDialog = new Lang.Class({
                                 x_align: St.Align.END,
                                 y_align: St.Align.START });
 
-        let messageLayout = new St.BoxLayout({ vertical: true });
+        let messageLayout = new St.BoxLayout({ vertical: true,
+                                               style_class: 'end-session-dialog-layout' });
         mainContentLayout.add(messageLayout,
                               { y_align: St.Align.START });