Report user idle times to logind
See https://lists.freedesktop.org/archives/systemd-devel/2019-May/042742.html for the rationale.
Logind offers a standard interface for querying system or session idle time, but it works only if the main operator of the session (usually the display server) tells the idle time to logind.
Implement this in libweston's logind support part and add the necessary core framework to track idle time to begin with.
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