Make PLY export cross platform
Make PLY export cross platform

Change-Id: I516c3abea0b62bd84f460ba5b4321eb1c0acc503
TODO: nicer UI button

IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense