[glade] add background class to GladeDesignLayout children

[Juan Pablo Ugarte <jpu src gnome org>]

commit b257bdcdf26a45efe6a891d479f1371619085ce6
Author: Lukas K <lu 0x83 eu>
Date:   Sat Oct 29 15:35:30 2016 +0200

    add background class to GladeDesignLayout children

 gladeui/glade-design-layout.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gladeui/glade-design-layout.c b/gladeui/glade-design-layout.c
index 17e9a60..ec97982 100644
--- a/gladeui/glade-design-layout.c
+++ b/gladeui/glade-design-layout.c
@@ -973,10 +973,13 @@ glade_design_layout_add (GtkContainer *container, GtkWidget *widget)
 {
   GladeDesignLayout *layout = GLADE_DESIGN_LAYOUT (container);
   GladeDesignLayoutPrivate *priv = layout->priv;
+  GtkStyleContext *context = gtk_widget_get_style_context (widget);
 
   priv->child_rect.width = 0;
   priv->child_rect.height = 0;
 
+  gtk_style_context_add_class (context, "background");
+
   gtk_widget_set_parent_window (widget, priv->offscreen_window);
 
   GTK_CONTAINER_CLASS (glade_design_layout_parent_class)->add (container,