Allow to override preprocess function in class which extends CRM_Event_Form_ManageEvent_Location

When i create a class which extends CRM_Event_Form_ManageEvent_Location and try to extend the preprocess function, i run into the message _blocks not defined.

To be clear i'm trying to implement following function
class CRM_Event_Form_ManageCursus_Location extends CRM_Event_Form_ManageEvent_Location {
function preProcess()

{ parent::preProcess(); // Implement custom code }

}

The reason this fails is inside the preprocess function of CRM_Event_Form_ManageEvent_Location, this calls a static preprocess function on CRM_Contact_Form_Location. In this function the following is defined.

if (in_array($className, array(
'CRM_Event_Form_ManageEvent_Location', 'CRM_Contact_Form_Domain')))

{ $form->_blocks = array('Address' => ts('Address'), 'Email' => ts('Email'), 'Phone' => ts('Phone'), ); }

Which hardcodes this to those two classes. A better fix would be to make this available for all classes, but i think it's most simple to start to allow this for subclasses. I'll create a PR.