Harmonize QPA TrayIcon showMessage parameter order

Parameters were misnamed in some places but still managed to pass them
through in the right order.  Rename to avoid confusion, and keep the
parameter order the same as in QSystemTrayIcon::showMessage.

Change-Id: Ib4c8fff15eff15034c4ea64757803b118e487144
Reviewed-by: Marco Martin <mart@kde.org>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>