Change QNSView convertFromEvent: arguemnts to QPointF

Change QNSView convertFromEvent: toWindowPoint: andScreenPoint:
arguments from QPoint to QPointF.

QWindowSystemInterface event handlers already use QPointF and
QPointF is needed for tablet support
(https://codereview.qt-project.org/#change,62740).

Change-Id: I6e5b2f19777fe24ae34e9e85028772e4be0a94f6
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>