Create factories for DoctrineModule form elements

As discussed in https://github.com/doctrine/DoctrineModule/issues/175.
This facilitates the use of `DoctrineModule\Form\Element\*` without worrying about the injection of `ObjectManager`. In MyFieldset, I need to simply do:
```php
class MyFieldset extends Fielset
{
    public function init()
    {
        $this->add([
            'name' => 'MyEntitySelect',
            'type'   => 'objectselect'
            'options' => [...],
        ]);
    }
}
```
It would be nice if in the future we refactor `DoctrineModule\Form\Element\*` to inject `ObjectManager` through the constructor.

If it's good to merge, I'll write docs and do the same thing in **MongoODMModule**.