Use StateItemInterface::isTransitionAllowed before executing state transition

Problem/Motivation

The code for checking whether transition can be applied was written before it was possible to check whether certain transitions are applicable. Given one can override workflow definition and introduce new states there's no point in hardcoding states in conditions but it's better to use the StateItemInterface::isTransitionAllowed method

Steps to reproduce

Proposed resolution

      if (in_array('mark_paid', array_keys($order->getState()
        ->getTransitions()))) {
        $order->getState()->applyTransitionById('mark_paid');
        $order->save();
      }

should become

      if ($order->getState()->isTransitionAllowed('mark_paid')) {
        $order->getState()->applyTransitionById('mark_paid');
        $order->save();
      }

Remaining tasks

User interface changes

API changes

Data model changes