previously Persistence::add() was not called when Model::persistence was set and it implied no Persistence::HOOK_AFTER_ADD hook was called

this PR fixes it by replacing Model::persistence property with persistence getter/setter

with the newly introduced persitence Model::setPersistence() setter, Persistence:add() call does not make any logical sense ( persistence can be set only once and never unset) and thus direct Persistence:add() call newly throws an exception

Here are several examples how to upgrade your code when setting persistence:

regex to locate old usage: (?<!\w)(db|persistence)(?!\w).*->add\(

 $m = new Invoice($db);
 // no change required

-$m = $db->add(new Invoice());
+$m = new Invoice($db);

 $m = new Invoice();
 ...
-$db->add($m);
+$m->setPersistence($db);

When accessing the persistence, Model::getPersistence() getter must be used and it throws when persistence is not set. To check if persistence is set, use Model::issetPersistence() method.

regex to locate old usage: ->persistence(?!\w)

-$m = new Invoice($this->persistence);
+$m = new Invoice($this->getPersistence());