Resolve "Update to OpenCV 4"

Closes #1

See merge request !1

Merge branch '1-update-to-opencv-4' of gitlab.fz-juelich.de:ped-dyn-emp/petrack into 1-update-to-opencv-4
Merging changes

And minor bugfix in tracker.cpp

me by now)

Solution for Crash, see #1

Now everyone should be able to build with OpenCV4

Won't work on Mikes Mac, since the binaries of OpenCV4 were compiled on
Windows.

supported from now on
By accident, also enabling background substraction code, that should be
enabled, but was disabled due to CV_MAJOR_VERSION == 3 Macro

Ref #1

(changed Order in .pro for Wiki-tut to work)