OCL: DFT

(in collaboration with @ElenaGvozdeva).

check_regression=*OCL*Dft*
test_filter=*OCL*Dft*
test_modules=core
build_examples=OFF
Merge after Itseez/opencv#3000

Future work:
- Enabling `double` support
- Minor optimization
I think `std::map<int, Ptr<OCL_FftPlan>>` will be better, if we need to search by `dft_size` as key.