QStateMachine: move QState::finished() emision into method.

By moving the code for QState::finished() emission into a virtual
method which also receives the QFinalState that caused the emission,
subclasses can hook in and do some extra processing.

Change-Id: Id19947c09e196a0df4edb87d71b74b0600c78867
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>