Reduce spin box step size for standby/suspend/off setting from 10 minutes to 1 minute.

This should make it easier to change the values for the average user.
Some users might be frustrated by 10 minute steps and think they can't use smaller steps otherwise.
It also saves the user from using the keyboard for changing the settings.

5 minutes might be a more practical value, but it is less intuitive for the average user.

Entering high values is probably more rare and can be done using the keyboard.
But the limit is currently at 120 minutes anyway.