bugfix for issue  #3126, added missing cv::Ptr<> types to gen_java.py

gen_java.py was modified to support additional types:

Ptr_StereoBM 
Ptr_StereoSGBM 
Ptr_FaceRecognizer
Ptr_BackgroundSubtractorGMG
Ptr_BackgroundSubtractorMOG
Ptr_BackgroundSubtractorMOG2

small bugfix in gen_javadocs.py, too