Refactor Form AddSelect CRM_Utils_Api::getEntityName($this)

We should try to remove this piece of magic from the addSelect method.
See http://forum.civicrm.org/index.php/topic,35984.0.html for some discussions. I'll add a PR, to try to get a first step in this direction. This would do something like this:

1) Adds an uninitialized class variable to CRM_Core_Form
2) Initialized the variable in the event form
3) Modifies addSelect method to check that variable before doing any of its voodoo.