Draft: main/postmarketos-ui-xfce4: save and restore brightness on boots
This MR adds tool brightnessctl to save and restore brightness on boots. More details can be found in the commit message.
To avoid issues with brightnessctl, which is a rather simple tool, the display power settings were simplified in repository "xfce4-phone" version 0.5.3:
Additionally the post-install script was cleaned-up.