Add a port of Photivo, a free and open source photo processor.

Unfortunately, I had to deprive it from using OpenMP features, since
`compiler:openmp' pulls in GCC and libstdc++ which causes linking
problems due to incompatible std::string ABI. Also, while Qt 4.x
should be supported, it does not build against, so sadly it's Qt 5.x
based only for now.

WWW: http://photivo.org/