Doc: Add "What's New in Qt 5.4" page.

-Feature list will be added once the feature freeze is in effect.
-page lists all known API additions to 5.4 and thus good for debugging
 documentation.

Change-Id: Iec6f9113500cce393c684519a77176c9446a672d
Reviewed-by: Martin Smith <martin.smith@digia.com>