Add support for maxThreads>1

[MarkRivers]

All plugins should support maxThreads>1 if it is feasible and if there would be a performance gain.

I don't see any reason why it should not be feasible for NDPluginBar. It only processes a single image at a time (unlike for example NDPluginProcess which needs to know about the previous image), and that should be able to be done in a thread-safe manner. OpenCV and zbar operations are likely to be time-consuming and a linear improvement in throughput with number of threads could be expected.

The OpenCV and zbar libraries both claim to be thread-safe.

[jwlodek]

I made a pull request replacing doCallbacksGenericPointer and adding support for maxThreads > 1 here: #30