(2.4) Backports from master branch (#7854)

To resolve undefined "Mat_to_vector_KeyPoint" error

This reverts part of commit 0db10a30

refactored; added Camera2, notify callbacks, front/back maxCamera sizes; disable new stuff if target API < 21

`-E touch` command doesn't create intermediate directories. We have to do it
manually using `file(MAKE_DIRECTORY ...)` command.

- Substituted HAVE_WINRT with WINRT
- Fixed compilation issues in ocl.cpp and parallel.cpp
- Fixed compiler issue for WP8: "C2678: binary '+' : no operator found which takes a left-hand - Fixed gitignore
- Added #ifdef HAVE_OPENCL to remove compiler warnings in ocl.cpp
- Used NO_GETENV similar to '3rdparty\libjpeg\jmemmgr.c;
- Added ole32.lib for core module (for WindowsStore 8.0 builds)
- Made OpenCV_ARCH aware of ARM
Signed-off-by: Maxim Kostin <v-maxkos@microsoft.com>

- added java wrapper
- disabled python wrapper

fixed dependency of stitching module on xfeatures2d module as optional
fixed the initModule_xfeatures2d function that was called in module stitching since it is in another namespace than the standard cv one.

If both Python 2 and Python 3 are found, then build bindings for both of
them during the build process.  Currently, one version of Python is
detected automatically, and building for the other requires changes the
CMake config.

The largest chunk of this change generalizes OpenCVDetectPython.cmake to
find both a Python 2 and Python 3 version of Python.  Secondly, the
opencv_python module is split into two modules, opencv_python2 and
opencv_python3.  Both are built from the same source. but for different
versions of Python.

It's not documented, and it does nothing unless CMake 2.0 compatibility
is enabled (and it isn't):

https://github.com/Kitware/CMake/blob/v2.6.0/Source/cmConfigureFileCommand.cxx

(cherry picked from commit b75cbfde)

Conflicts:

	cmake/OpenCVModule.cmake

Conflicts:
	modules/java/generator/src/cpp/VideoCapture.cpp