Added notifications on changing brightness, will stack by default to
show the current level before disappearing.
The script to change and notify can be run through the command
`control-brightness`, and is bound to the sxhkd media key shortcuts for
brightness changes.
Removed the brightness indicator from polybar since it should be easy
enough to check current brightness by doing a quick higher-lower and
that's it.
Kept the module in polybar configuration script since I might reconsider
its removal if it turns out to have been useful.
