Unify primary key string and array representations

* Add getPKString and getPKArray methods on every propel object
	* These return strings/arrays even if the original PK was not composite or did not consist of strings
* Util::idForObject is deprecated for propel object instances
* TODO: The tagged item id of tag instances is currently a number which does not work for composite or string pk’s (meaning: we can’t currently tag language objects, strings, …)