fixing cap_pvpapi interface

This PR tries to fix the following 3 reported issues
- http://code.opencv.org/issues/3946
- http://code.opencv.org/issues/3947
- http://code.opencv.org/issues/3948

A complete remake of the PvAPI API interface solving bugs but also
- allowing the use of MANTA type cameras
- allowing multiple AVT cameras at the same time
I would suggest informing the user why the function failed as well:
fprintf(stderr, "Pixel format %s not supported; only modes Mono8, Mono16 and Bgr24 are currently supported\n", pixelFormat);
return false;