Ensure the QLineEdit::returnPressed() signal is still emitted

The signal needs to be emitted directly as the event is not passed to
the QLineEdit if the QSpinBox gets the Key_Return. Since this signal
may be relied upon then we ensure it is emitted directly.

Change-Id: I17cdec62c9f995bacfd7d3cc66d6324f26c84c67
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>