Callbacks don't stack

[jesse-r-s-hines]

Callbacks set on the "base case" definition will be overridden by callbacks added to any groups on the same model, meaning that the first callback won't be called. Since groups definitions build on the "base case" I would expect the callbacks to stack similarly.

For example:

$fm->define("User")->setDefinitions([
    // definitions ...
])->setCallback(function ($object, $saved) {
    $object->email = "user@example.com";
});

$fm->define("mygroup:User")->setDefinitions([
    // more definitions ...
])->setCallback(function ($object, $saved) {
    $object->address = "some address";
});

Then if I create a plain User, fields email will be set and address will be blank, as expected...

$user = $fm->create("User");
$user->email; // "user@example.com"
$user->address; // null

However if I use the mygroup:User factory the first callback isn't called, only the second

$user = $fm->create("User");
$user->email; // null
$user->address; // "some address"

This requires duplicating any logic that was in the User callback in all groups.

It would be useful to have a way to "stack" the callbacks, either by modifying setCallback() to add a callback to a list instead of replacing any previous callbacks, or making an new addCallback() method that does that.