Do not define the constant ICINGAWEB_LIBDIR

This constant should not be needed anywhere.
Level up index.php

This is the preparation for having the Icinga library in PHP's include path.
Fix ICINGAWEB_APPDIR location if the Icinga library is in PHP's include path
Prepare icingacli for having the Icinga library in PHP's include path
Define the ICINGAWEB_VENDORS contant
No longer make ICINGAWEB_APPDIR configureable