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
`grayframe->widthStep` is in bytes, right? In this case you need
```
grayframe->widthStep = (int)frameWidth*2;
```