Option to run process with a nice level (Linux), or lower priority (Windows)

[planetrocky]

I've been using this to run Kometa on my main machine in the background. On large libraries it takes a while, and my machine is pretty ancient.

Windows I've been running BELOW_NORMAL_PRIORITY_CLASS for a while now, and everything is OK.

This is a proof of concept, it should be controlled by a setting etc, but in case anyone else finds this useful :)

def low_priority():
    """ Set the priority of the process to below-normal."""

    import sys
    try:
        sys.getwindowsversion()
    except AttributeError:
        isWindows = False
    else:
        isWindows = True

    if isWindows:
        import win32api, win32process, win32con

    # priorityclasses:
    #  win32process.IDLE_PRIORITY_CLASS,
    #  win32process.BELOW_NORMAL_PRIORITY_CLASS,
    #  win32process.NORMAL_PRIORITY_CLASS,
    #  win32process.ABOVE_NORMAL_PRIORITY_CLASS,
    #  win32process.HIGH_PRIORITY_CLASS,
    #  win32process.REALTIME_PRIORITY_CLASS]

        win32process.SetPriorityClass(win32api.GetCurrentProcess(), win32process.BELOW_NORMAL_PRIORITY_CLASS)
    else:
        import os

        os.nice(10)

if __name__ == "__main__":
    low_priority()
    try:
        if run_args["run"] or run_args["tests"] or run_args["run-collections"] or run_args["run-libraries"] or run_args["run-files"] or run_args["resume"]:
            process({"collections": run_args["run-collections"], "libraries": run_args["run-libraries"], "files": run_args["run-files"]})
        else:

[planetrocky]

This has been working for me for the past 4-5 months, so I've added a run command `--nice`` and submitted as a PR #2304