Simple daemon

Closes #3 (closed)

The Background feature is implemented by keeping the app (including the qml engine) running, when all windows are closed. This allows us to handle dbus and notification events in qml.

An autostart option is added via a systemd user service. Autostart and background mode are fully separate - so it is possible to just start the app at boot without keeping it in the background. After autostart, the app will only be hidden if background-mode is enabled and storage encryption is disabled.

The related feature and config-option can be disabled using the "harbour" feature flag (except for dbus-single-instance management, which we might need for custom url handlers as well).

To keep the systemd service out of the rpm if "harbour" is requested, build.rs contains some conditional copying and filtering of the rpm .spec file. This might be overkill, since the captcha locks us out of harbour for the foreseeable future, but on the other hand this provides infrastructure that could be easily adjusted to other non-harbour features like a share system.