OpenNI 2 grabber

Experimental version of an OpenNI 2.x grabber.

I tested the existing OpenNI 1.x grabber under VS2010 and did not find any obvious regressions or errors with this code in place, so the two should be able to coexist as long as they are not built at the same time. There is a CMake check preventing this.

Several apps and example programs break encapsulation and directly use OpenNI 1.x data structures, so they will not work with the new grabber until this is fixed.

Linux compatibility has not been tested, but I expect the new grabber will fail to build until FindOpenNI2.cmake is updated with the correct default paths.