Lomiri Greeter: launch as systemd service and avoid run-time conditions with failing lomiri-keyboard

Starting with lomiri(-greeter) 0.2.1-4, the greeter is usable as LightDM greeter using LightDMs wayland compositor feature.

The launch-up happens in /usr/bin/lomiri-greeter-wrapper. It is assumed that lomiri-greeter can only run on LightDM as single-head display manager.

So, we can use systemd --user to launch all the services and make use of the DBus activation environment feature. At the moment, Ayatana Indicators and Lomiri (Maliit) Keyboard get loaded via systemd --user whereas lomiri gets executed manually. This can lead to runtime issues and needs to be improved (i.e. launch lomiri as systemd --user service).

When changing this, it needs to be assured that the lomiri-keyboard.service gets taken down before lomiri shuts down its Wayland display socket.