Move system update flow to GO, with a CLI (convenient AIO tool) and
server integration. All lifecycle, scheduling, execution occurs on
backend side.
Run some backends via pkexec, some via terminal like paru/yay.
Incorporate flatpak as an option to update.
Add terminal override setting in GUI, in addition to $TERMINAL env
variable.
fixes#2307fixes#822fixes#1102fixes#1812fixes#1087fixes#1743
