Move index.php content to App->runFrontend

- Replace all x() by !empty() in App
- Replace all killme() with exit()

src/Core/System.php

@@ -142,7 +142,7 @@ class System extends BaseObject
 				'$description' => defaults($description, 'description', '')]);
 		}
 
-		killme();
+		exit();
 	}
 
 	/**