Split GPU module into several submodules

Created next gpu modules:
* gpuarithm (arithmetics)
* gpulegacy (legacy NCV & NPP API)
* gpufilters (image filtering)
* gpuwarping (image warping)
* gpuimgproc (image processing)
* gpubgsegm (background segmentation)
* gpucodec (video encoding/decoding)
* gpufeatures2d (2d feature detection and matching)
* gpuoptflow (optical flow)
* gpustereo (stereo correspondence)
* gpu (unclassified functionality)