Manage set priority

Allow ka-lite process priority to be lowered.
Intended implementation is to reduce priority of some background processes, particularly on Raspberry Pi #839

Lowers the CPU scheduler priority, and if "Lowest" mode is chosen, then IO priority is also lowered.  Lowest priority will only be scheduled by the OS if the IO channel is idle.

Not implemented for Windows in this PR
Requires psutil on *nix
Existing psutil installation is broken due to binary blob, so psutil python-packages is dropped by this PR  #750