Add ocl::sortByRow()

The added method ocl::sortByRow() sorts the columns of an oclMat by the values in a specific row. It can handle single and multi channel matrices.
A possible use case is sorting keypoints by their response row and keep just the n best keypoints.
Using the already available function ocl::sortByKey() isn't reasonable here, because it allows only 1D arrays and is intended to have separate keys and values.