Update minimum required Qt version.

We require Qt 5.2 for full functionality nowadays.
It is possible to use Qt 4.8.x with restrictions.
(Qt 5.0 and 5.1 do not work because of problems with embedding
Qt Quick 2 and other Qt Quick related issues (e.g. QTCREATORBUG-10273)

Change-Id: Ib41576e8b8e3727e87141eaf0351d63158730524
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>