QSplashScreen - add message function

There is no point in not being able to get the message.
Sometimes it can be the case that we just want to append a dot
(or something else) on the existing message.

Change-Id: I171495f1edc568ae1654f43b3cf734a4eb67d374
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>