We should make DETECT_PERIODICALLY default

[ayberkozgur]

We should make DETECT_PERIODICALLY the default detection trigger because with DETECT_ONLY the "doesn't care about tweaking settings" user is missing out on tracking goodness. He/she will not even notice we have tracking at all.

[qbonnard]

Adding to 3.0 since it's kind of backward incompatible, and tracking is still a bit new.

[qbonnard]

I remember that we already discussed this, but can't find the discussion again. One point was that for unrelated image (e.g. batch processing of input files, or in the tests), that's bad.

[ayberkozgur]

Batch processing is actually a valid point but I would say real-time usage would be much more common than batch processing use cases.