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.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4986

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