[BUGFIX] Invalid shortcut target on translated pages

A feature to resolve shortcut links directly in menu rendering
does not consider the overlay behaviour of the "shortcut" value.

TSFE first uses the "shortcut" value of the original/default
page and then serves for possible translated alternatives
(TypoScriptFrontendController::checkTranslatedShortcut()).
In menu rendering it's the other way round, the translated
overlay (if any) is used directly - the fallback to use the
"shortcut" value of the original/default page is not implemented.

This change introduces the fallback when rendering menus, which
will take the "shortcut" value of the default language if the
value in the overlay is empty.

Change-Id: I26a9eb4813c6b99327043b0a764ec7ff80b6a905
Fixes: #36822
Releases: 6.2, 6.1
Reviewed-on: http://review.typo3.org/27676
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>