Move invoice/shipment/creditmemo out of Abstract.php

[Schrank]

example: \FireGento_Pdf_Model_Engine_Abstract::insertHeader()

    if ($this->getMode() == 'invoice') {
        $title = 'Invoice';
    } elseif ($this->getMode() == 'shipment') {
        $title = 'Packingslip';
    } else {
        $title = 'Creditmemo';
    }