Regression of 46666a77.
The code in MenuBarManager::FillMenuManager merges the
addon menu before .uno:WindowList, which doesn't exist
in the start center since that commit.

As a last resort, let's also check for .uno:HelpMenu
(assuming that it's never placed before the Window menu).

Change-Id: If45eebe4351c40d8ed69daba527844ffc02e8458
Reviewed-on: https://gerrit.libreoffice.org/18729Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>