Export of CPU usage for selected processes and threads

[whimboo]

Once we have fixed issue #24 we could think about exporting the CPU usage for all selected processes and threads. When doing that including mean values for each process and thread would be very handy.

[whimboo]

I wonder if this could be done in a Gecko profiler supported format, so that it could be imported on https://profiler.firefox.com. Might need some investigation.