QWinTaskbarButton: minor tweaks

Make the constructor's parent argument optional, and do not
pass a pointer to a half constructed object to the private
implementation.

Change-Id: I48df98f8d55ec49fd8f155b14167038055712cfc
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>