Simplify gnome-power-manager back light settings and make less intrusive

Ubuntu has an annoying behavior, gnome-power-manager periodically overrides screen back light brightness during normal activity. As now, GPM should manage the back light, according to whether in battery or mains mode. When back light brightness is set manually (e.g. with a keyboard shortcut such as Fn+↑) that level should be set as the new default for the current mode. This proposal does not cover behavior when the computer is idle.