Unify the \since base line version for qdoc

The first stable API release is Qt 5.8. It makes no sense to document
the changes before 5.8. Furthermore the meaning of \since 5.8 before this
tag was rather unknown. It could have meant the function was changed or added.
This double meaning disappears once a compatibility statement is in place.

Change-Id: I047e15bdd92a8c1034947b9b6d03439a293af318
Reviewed-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>