Basic doxygen documentation support

- updated existing Doxyfile.in
- added corresponding cmake instructions
- added some specific files (layout, icon)
- clean existing doxygen warnings

Conflicts:
	CMakeLists.txt
	doc/CMakeLists.txt
	modules/core/include/opencv2/core.hpp
	modules/core/include/opencv2/core/base.hpp
	modules/core/include/opencv2/core/cuda.inl.hpp
	modules/core/include/opencv2/core/mat.hpp
	modules/core/include/opencv2/core/matx.hpp
	modules/core/include/opencv2/core/types.hpp
	modules/flann/include/opencv2/flann/lsh_table.h
	modules/imgproc/include/opencv2/imgproc.hpp

modules/gpu/include/opencv2/gpu/gpu.hpp

@@ -537,7 +537,7 @@ CV_EXPORTS void log(const GpuMat& a, GpuMat& b, Stream& stream = Stream::Null())
 //! supports all, except depth == CV_64F
 CV_EXPORTS void pow(const GpuMat& src, double power, GpuMat& dst, Stream& stream = Stream::Null());
 
-//! compares elements of two arrays (c = a <cmpop> b)
+//! compares elements of two arrays (c = a \<cmpop\> b)
 CV_EXPORTS void compare(const GpuMat& a, const GpuMat& b, GpuMat& c, int cmpop, Stream& stream = Stream::Null());
 CV_EXPORTS void compare(const GpuMat& a, Scalar sc, GpuMat& c, int cmpop, Stream& stream = Stream::Null());
 
@@ -2264,6 +2264,7 @@ public:
      * model.
      * @param frame        Input frame
      * @param fgmask       Output mask image representing foreground and background pixels
+     * @param learningRate determines how quickly features are “forgotten” from histograms
      * @param stream       Stream for the asynchronous version
      */
     void operator ()(const GpuMat& frame, GpuMat& fgmask, float learningRate = -1.0f, Stream& stream = Stream::Null());