If you want to alter the value of a Spin Value relative to its
current value, then this ffunction can be used.
The direction paramenter is one of SPIN_END , SPIN_HOME , SPIN_PAGE_BACKWARD , SPIN_PAGE_FORWARD , SPIN_STEP_BACKWARD , SPIN_STEP_FORWARD and SPIN_USER_DEFINED
GTK.SpinStepForward and GTK.SpinStepBackward change the value
of the Spin Button by the amount specified by increment, unless
increment is equal to 0, in which case the value is changed by the
value of step_increment in theAdjustment.
GTK.SpinPageForward and GTK.SpinPageBackward simply alter the
value of the Spin Button by increment.
GTK.SpinHome sets the value of the Spin Button to the bottom of the
Adjustments range.
GTK.SpinEnd sets the value of the Spin Button to the top of the
Adjustments range.
GTK.SpinUserDefined simply alters the value of the Spin Button by
the specified amount.