Do not extend Zend_Config in Icinga\Application\Config

refs #7147
fixes #7580
Fix that current(), key(), etc does not work with Config objects
Fix that an error is thrown when reading INI files without trailing newlines
Prefer Icinga\Application\Config instead of Zend_Config

refs #7147
Merge branch 'master' into bugfix/drop-zend-config-7147
Merge branch 'bugfix/drop-zend-config-7147'

fixes #7147