Fix the build: add missing #includes

<QWidget> for QWidget, <QDataStream> for QDataStream and <QMetaObject>
for Q_GADGET (technically, qobjectdefs.h)

Change-Id: Ia7e8b12ff4c67827bb7782a0337714e330de1db1
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>