Doc: Integrated Qt 5.4 features wiki into What's New in Qt 5.4 page.

-content taken from the wiki page.

Task-number: QTBUG-40758
Change-Id: Ic2d847162466754dd93d4b2c47465fb3dad8b263
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>