Q(Double)SpinBox: add signal returnPressed()

QSpinBox (QDoubleSpinBox) should provide a signal returnPressed(), just like QLineEdit does.

Pressing the return/enter key in spinboxes is often used to confirm+trigger any actions.

setKeyboardTracking(false) + signal valueChanged/textChanged is not convenient for this purpose, because the signals are also emitted when keyboard focus is lost, or when other spinbox functionality is used (in which case the entered value usually can/should not be interpreted as "confirmed").

Q(Double)SpinBox internally uses a line edit, so all that (likely) needs to be done is to pass QLineEdit::returnPressed() on.