Fix for issue #5699 - Disable label position caching for Zend\View\Helper\FormRow::__invoke()

- Reverted #4771 so `__invoke()` does not cache label position anymore
- Added new method `setDefaultLabelPosition` which is cached and reused by `Zend\View\Helper\FormRow`
- Added new method `getDefaultLabelPosition` for easy debugging purposes
- Added tests for the new functionality