Make time optional for Trackers

[soelderer]

Removes time from baseHeaders and RecordRow. This removes the dependency on Unity API in the base class, which allows RecordRow to be called outside the main tread. It also allows for non-time-based Trackers (e.g. frame count or event based). For the provided prototypes MouseScreenTracker, MouseWorldTracker and PositionRotationTracker, the timestamp is added in GetCurrentValues instead of RecordRow to provide the same functionality as before.

This fixes #181