use FormFlowInterface in TwigExtension instead of concrete class

[acrolyt]

If one wants to overwrite the FormFlow class or implement his own implementation, the original TwigExtension will work out of the box

[coveralls]

Changes Unknown when pulling e0b0605 on acrolyt:use-interface-in-twig-extension into ** on craue:master**.

[coveralls]

Changes Unknown when pulling e0b0605 on acrolyt:use-interface-in-twig-extension into ** on craue:master**.

[coveralls]

Changes Unknown when pulling e0b0605 on acrolyt:use-interface-in-twig-extension into ** on craue:master**.

[coveralls]

Changes Unknown when pulling df75c7d on acrolyt:use-interface-in-twig-extension into ** on craue:master**.

[coveralls]

Changes Unknown when pulling df75c7d on acrolyt:use-interface-in-twig-extension into ** on craue:master**.

[craue]

It's not that easy because methods getDynamicStepNavigationInstanceParameter and getDynamicStepNavigationStepParameter (used in FormFlowUtil) are not part of FormFlowInterface.