Respect existing backslashes

As we already might be in a namespace, some legacy classnames might be already escaped. We have to check that in all sniffs and handle accordingly:

-    protected function setViewConfiguration(\Tx_Extbase_MVC_View_ViewInterface $view)
+    protected function setViewConfiguration(\\TYPO3\CMS\Extbase\Mvc\View\ViewInterface $view)
     {