resize-popup: set a bigger margin

Since we're going to use the tooltip's rounded corners we need a little
bit more of margin (which wasn't a bad idea even with the frame).
Also, don't use GtkMisc for this anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=692741

src/ui/resizepopup.c

@@ -84,7 +84,7 @@ ensure_size_window (MetaResizePopup *popup)
                             TRUE);
   
   popup->size_label = gtk_label_new ("");
-  gtk_misc_set_padding (GTK_MISC (popup->size_label), 3, 3);
+  g_object_set (popup->size_label, "margin", 6, NULL);
 
   gtk_container_add (GTK_CONTAINER (popup->size_window), popup->size_label);