Add human readable timespan representation to outputs.

If a DateTime object is shown to the user (log, tray, ...) it should be appended by a human readable representation like 4 minutes ago or 3 weeks ago.

Implementation suggestions:

• SO: Human-readable date formats
• SO: How to produce “human readable” strings to represent a TimeSpan
• SO: Calculate relative time in C#
• C# program that displays pretty dates

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