Provide a way to specify resulting sis name for "make sis" and createpackage.pl

We'd like to be able to specify the name of the created file for "make sis" from Qt Creator for mainly two reasons:

1) To guard against changes of the default in the scripts. Exactly this has happened between Qt 4.6.1 and 4.6.2, in 4.6.1 the created packages contains the information about platform and target, in 4.6.2 that was removed. Managing changed defaults between different versions is a pain

2) This one is not yet fully evaluated, but we might want to give users means to specify a name, so if they have different signing configurations, they can give the result comprehensible names.