Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.

Turns out, you can't use preprocessor directives inside macro arguments.
Who'd have thought?

Now it allows choosing between arbitrary implementation variants.

* disabled warnings from thrust
* fixed warnings from ts_gtest.h
* possibly fixed superres compilation in Debug mode on Windows

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.