Include the backend module pagetree in a different way

Backend modules below the WEB-module always include the common
TYPO3-pagetree.

As Goobi.Presentation has its own main module we have to add the pagetree
manually. This is done with t3lib_extMgm::addNavigationComponent which is
working in all supported TYPO3 versions from 4.5 to 6.2.