Implement brightnessStep logic.

Introduce brightnessStep, brightnessStepMax, brightnessStepChanged, setBrightnessStep D-Bus endpoints in org.kde.Solid.PowerManagement.Actions.BrightnessControl.
Introduce correspoding D-Bus endpoints in org.kde.Solid.PowerManagement.Actions.KeyboardBrightnessControl.

Use step-based logic for increase/decrease brightness actions.

This makes the increase/decrease brightness actions behave as if the brightness steps are pre-calculated.

REVIEW:119597