make 'listeners' key can be configured outside application.config.php
because in ZF2 Skeleton App use : Zend\Mvc\Application::init(require 'config/application.config.php')
fix non context $this
use array_merge application config with other config to avoid bc
only merge with key 'listeners'
enhance variable name and precedence merge
Merge pull request #5612 from samsonasik/listener/outside.application.config

make 'listeners' key can be configured outside application.config.php
Merge branch 'feature/5612' into develop

Close #5612