Use clFinish to gurantee commands completed, instead of waiting for events.
Signed-off-by: Wu Zhiwen <zhiwen.wu@intel.com>

no changes in code

The same code was repeated several time for different data types, so
it was extracted as a templated function to improve maintability and
make a code more clear.

The entire AssetsLibrary framework is deprecated since iOS 8.0. The code
used in the camera example code can use UIKit to save videos to the
camera instead, which allows to avoid linking with PhotoKit instead to
prevent increasing the iOS deployment target.

Additionally adds a tutorial on how to use the tag file in other
projects to link back to the OpenCV documentation.

The tutorial is also the only place which links to the tag file as of
now.

For more information on tag files and external documentation, check
https://www.stack.nl/~dimitri/doxygen/manual/external.html