Histogram support for each GPU

[JeremyMain]

Current versions only compute histograms for the first GPU, now that CPU/RAM and other non-GPU graphs have been isolated it will be easier to compute and display the histogram information for each GPU.

[JeremyMain]

I am seriously considering disabling the histogram function in the next release and re-enable it in the following release as there is a significant amount of rework that needs to be done. v1.07b is packed full of new and useful features and wants to be used, not delayed.