Refactoring of the xbacklight

Hello,

This MR:

• makes fading as smooth as possible when fps or steps are not provided
• switches error output from stdout to stderr
• adds the AcpilightError exception and removes the error function
• moves all methods into classes
• changes a way of controller obtaining
• applies isort
• fixes flake8 warnings
• moves argument parsing into a separate method
• makes some other changes

I hope you decide to merge it. Thanks!