Allow users to select QueryPanel search engines from a list

Summary: Ref T4986. Instead of requiring users to know the name of an application search engine class, let them select from a list.

Test Plan:
Created a new panel.

{F165468}

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9500

src/applications/files/query/PhabricatorFileSearchEngine.php

@@ -3,6 +3,10 @@
 final class PhabricatorFileSearchEngine
   extends PhabricatorApplicationSearchEngine {
 
+  public function getResultTypeDescription() {
+    return pht('Files');
+  }
+
   public function getApplicationClassName() {
     return 'PhabricatorApplicationFiles';
   }