xsession: remove duplicate sourcing of /etc/profile
remove redundant if
xsession: add config option to let systemd handle lid/power events
add e17 desktop manager

desktop-manager is a bit misleading in this case as there is no
session yet and most apps are still missing. This can eventually
grow further once more e17 apps get packaged for nix.

However, for now, I need to initialize some e17 dbus services to
have the "terminology" terminal emulator provide gfx previews.