data: Use the correct name for parent widgets in templates

Recent versions of GTK+ started complaining about this, for example:
(totem:5745): Gtk-CRITICAL **: Error building template class 'TotemMainToolbar' for an instance of type 'TotemMainToolbar': Template parent type `GtkBox' does not match instance parent type `GtkHeaderBar'.

https://bugzilla.gnome.org/show_bug.cgi?id=729479