Add a function to get the selection of a clipboard

This was requested in
http://bugzilla.gnome.org/show_bug.cgi?id=730821

gtk/gtkclipboard.c

@@ -2208,3 +2208,21 @@ _gtk_clipboard_store_all (void)
   g_slist_free (displays);
   
 }
+
+/**
+ * gtk_clipboard_get_selection:
+ * @clipboard: a #GtkClipboard
+ *
+ * Gets the sleection that this clipboard is for.
+ *
+ * Returns: the selection
+ *
+ * Since: 3.22
+ */
+GdkAtom
+gtk_clipboard_get_selection (GtkClipboard *clipboard)
+{
+  g_return_val_if_fail (GTK_IS_CLIPBOARD (clipboard), GDK_NONE);
+
+  return clipboard->selection;
+}