[pitivi/gtktimeline] mainwindow: Fix applying mnemonic from button label

[Thibault Saunier <tsaunier src gnome org>]

commit fed8dc3fb518f5957c40e18c2082ee64bf72aeb4
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Wed May 13 09:02:42 2015 +0200

    mainwindow: Fix applying mnemonic from button label

 pitivi/mainwindow.py |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/pitivi/mainwindow.py b/pitivi/mainwindow.py
index 6c2f67a..8fb3471 100644
--- a/pitivi/mainwindow.py
+++ b/pitivi/mainwindow.py
@@ -428,12 +428,14 @@ class PitiviMainWindow(Gtk.ApplicationWindow, Loggable):
         self.undo_button.set_always_show_image(True)
         self.undo_button.set_label(_("Undo"))
         self.undo_button.set_action_name("app.undo")
+        self.undo_button.set_use_underline(True)
 
         self.redo_button = Gtk.Button.new_from_icon_name(
             "edit-redo-symbolic", Gtk.IconSize.LARGE_TOOLBAR)
         self.redo_button.set_always_show_image(True)
         self.redo_button.set_label(_("Redo"))
         self.redo_button.set_action_name("app.redo")
+        self.redo_button.set_use_underline(True)
 
         separator = Gtk.Separator()