Fix include of imgcodecs

fix compiling reg module after separating imgcodecs from highgui

added samples to use retina in its basic form and for HDR tone mapping

More abstract tracker declaration

This contribution changes the way how trackers should be implemented by
programmer. It should be noted first, that from the user prospective,
Tracker::create() method still can be called with the same effect.
However, there's no more thing as a "constructor" for Tracker, they
should be created via static functions, e.g.
    static Ptr<TrackerMIL> createTracker(const TrackerMIL::Params &parameters=TrackerMIL::Params());

From the programmer's perspective now for every tracker what is declared
in tracker.hpp header is more a "interface", while the real
implementation class can (and has to) be implemented in .cpp file.

Changes in documentation are also included.

ADAS: training app, stumps and waldboost