- made some of dependencies explicit
- removed dependencies to highgui and some other modules where possible
- modified some samples to build without modules

Currently if you are building OpenCV with contrib under ios, not as framework, this can leads to errors in config.
I suppose it would be better if xobjdetect module app will be optional with the BUILD_opencv_apps just like others tools in OpenCV

Fixed warnings for XCode 7.1.1 and cmake 3.3.2, rewrote one method in sliency to avoid clang 7.0.0 crash