Reworked & simplified based on feedback from DDCUtil author in D5381.
Fixed CMakeList to add definitions for use in the code.
Updated the code to use the the ddcutil 0.9.5 C API
- multiple DDC-capable monitors connected will be used, however only one slider is showed. All monitors will be affected with the same brightness value.
- ddcutil has to be setup so that it can be used as user (1. load i2c-dev on boot, 2. udev rule to affect /dev/i2c-* nodes to the i2c user group, 3. add your user to the i2c group.)