added optional dependency stitching->nonfree. When nonfree is available, make sure linker does not throw it away.

lot's of changes; nonfree & photo modules added; SIFT & SURF -> nonfree module; Inpainting -> photo; refactored features2d (ORB is still failing tests), optimized brute-force matcher and made it non-template.

 [~] Automatically tracked dependencies between modules
 [+] Support for optional module dependencies
 [+] Options to choose modules to build
 [~] Removed hardcoded modules lists from OpenCVConfig.cmake, opencv.pc and OpenCV.mk
 [+] Added COMPONENTS support for FIND_PACKAGE(OpenCV)
 [~] haartraining and traincascade are moved outside of modules folder since they aren't the modules

Made changes in the stitching log macros: now the function stitchingLogLevel() may be used to make the stitching classes more/less verbose.

Avoid duplicated surf extraction step when same parameters are used for detection and descriptors calculation

added first version of public stitching API, added simple sample which uses that API, old sample renamed to stitching_detailed

switched to gpu::remap in opencv_stitching