Option to extract a number of video frames as a function of video duration

[lfcnassif]

As discussed in other issues #1081 #1102, this could improve search results of a number of video tasks (nudity and faces detection, future similarity search, OCR and photoDNA and others).

extractFramesMillisecondInterval = 1000 would be a good default value for pedo and forensic profiles and disabled for others?

Or should we make some kind of attenuation targeting huge videos, maybe using a logarithmic or similar function applied to video duration?

[lfcnassif]

I'm increasing the priority of this, this can be useful to #1472.

[lfcnassif]

@tc-wleite I'm coding to change just the number of subitems extracted (if enabled) to don't mess the mosaic already generated, ok?

[wladimirleite]

@tc-wleite I'm coding to change just the number of subitems extracted (if enabled) to don't mess the mosaic already generated, ok?

Sure! I think it should be fine, as the algorithm that generates the mosaic can receive more images than it actually uses.
I will be out next week, but I can take a look in the changes and test if you want.