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 haven't tested the mono16 yet but if you look at the cvCreateImage() function you will notice that it is created with pixel size of 16 bits (IPL_DEPTH_16) and not 8-bits so the width step is still 1, which is 16 bits, or 2 bytes. I'll test this over the weekend.