[StBoxLayout] Add st_box_layout_get_n_children

https://bugzilla.gnome.org/show_bug.cgi?id=600734

src/st/st-box-layout.c

@@ -1385,3 +1385,15 @@ st_box_layout_destroy_children (StBoxLayout *self)
 {
   st_box_layout_internal_remove_all (self, TRUE);
 }
+
+/**
+ * st_box_layout_get_n_children:
+ * @self: a #StBoxLayout
+ *
+ * Returns the number of children in this box.
+ */
+guint
+st_box_layout_get_n_children  (StBoxLayout *self)
+{
+  return g_list_length (self->priv->children);
+}

src/st/st-box-layout.h

@@ -93,6 +93,8 @@ void     st_box_layout_remove_all     (StBoxLayout *box);
 
 void     st_box_layout_destroy_children (StBoxLayout *box);
 
+guint    st_box_layout_get_n_children  (StBoxLayout *box);
+
 G_END_DECLS
 
 #endif /* _ST_BOX_LAYOUT_H */