modules/videoio/src/cap_openni.cpp: fix const-ness related compile errors

Commit 854a722c introduced compile errors (linux, gcc 4.8.3) because CvCapture_OpenNI::getProperty was const-ified, while the methods it calls - CvCapture_OpenNI::getImageGeneratorProperty(), CvCapture_OpenNI::getDepthGeneratorProperty(), and CvCapture_OpenNI::getCommonProperty() were not.

This commit const-ifies the above three methods. In addition, a const_cast is needed for the argument of
depthGenerator.GetAlternativeViewPointCap().IsViewPointAs() in getDepthGeneratorProperty().