Move declaration of IVideoCapture class into precomp.hpp, make some methods of IVideoCapture pure virtual

Conflicts:
	modules/contrib/src/basicretinafilter.hpp
	modules/contrib/src/magnoretinafilter.hpp
	modules/contrib/src/parvoretinafilter.hpp
	modules/contrib/src/retinacolor.hpp
	modules/gpu/include/opencv2/gpu/gpu.hpp
	modules/highgui/src/cap_tyzx.cpp
	modules/ocl/perf/perf_filters.cpp
	modules/ocl/src/haar.cpp
	modules/ocl/src/opencl/haarobjectdetect.cl
	modules/ocl/test/test_filters.cpp
	modules/ocl/test/test_objdetect.cpp