Move examples into a dedicated 'serialport' directory. This is
in line with what the other modules do, and makes sure that
the examples show up  in a sensible place even for the 'combined'
source packages.

Task-number: QTBUG-30912
Change-Id: Iefa2b634df3d2eb34f655b34f6fb24a224b78869
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>