Settings.hasTouchScreen checks the QTouchDevices

This should be more reliable than making assumptions based only on
the platform screen.  This also means that ultrabooks, desktops etc.
which have multiple input devices including a touchscreen will
have touch optimizations.  However due to QTBUG-36007 it's still necessary
to check for the platforms that do not create QTouchDevices properly
at startup.

Change-Id: I6cc0408b472af16580e91509eb4cbc420a263871
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>