Add support for routes in form redirect.

Follow-up from #1987806: Convert search_view() to a new style controller.

Problem/Motivation

Currently forms using 'redirect' with system URI to redirect to certain page after processing/complete form submission.
e.g. $form_state['redirect'] = $form_state['action'] . '/' . $keys; in SearchForm::submit()

Proposed resolution

Add an option to use routing system to do this redirect. So instead of url, it might be route_name?

Remaining tasks

Discuss
Issue patch
Test

User interface changes

NO

API changes

May be - Depending on the implementation

Related Issues

https://drupal.org/node/1987806#comment-7936965

Originally reported by @dawehner at #1987806-36: Convert search_view() to a new style controller