no data frames

[Stephen Gatesy]

Designate some frames as never having data (tracked or interpolated). Tracker often follows markers for several frames beyond calibrated or imaged area and require cleanup.

[Benjamin Knorlein]

fixes #82 : Points can be set untrackable when Ctrl (or Command) + U is clicked. To set it to trackable again, point data has to be deleted by using Delete or Backspace

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