FilterEditor: Use a new url from request when redirecting searches

Creating a new url has the benefit that all framework params are
still there. `$this->url()` however returns a url that's already
mangled and parameter preservation does not work for framework
params.

This is not quite the correct fix. But the entire parameter
handling here is way too convoluted.

library/Icinga/Web/Widget/FilterEditor.php

@@ -285,12 +285,10 @@ class FilterEditor extends AbstractWidget
                 }
             }
 
-            $url = $this->url()->setQueryString(
-                $filter->toQueryString()
-            )->addParams($preserve);
-            if ($modify) {
-                $url->getParams()->add('modifyFilter');
-            }
+            $url = Url::fromRequest()->onlyWith($this->preserveParams);
+            $urlParams = $url->getParams();
+            $url->setQueryString($filter->toQueryString());
+            $url->getParams()->mergeValues($urlParams->toArray(false));
             $this->redirectNow($url);
         }