Correctly nice and ionice the worker processes.

This had stopped working.
Unfortunately it doesn't seem to help anyway.
Ideally the responsiveness of the interface is the same, regardless
of whether the workers are paused or not. Atm its much better when
they are paused.

worker.py

@@ -7,15 +7,11 @@ import asyncio
 import os
 import signal
 
-import psutil
-
 import tools
 
 
-def _make_process_nicest(pid):
-    process = psutil.Process(pid)
-    process.nice(19)
-    process.ionice(psutil.IOPRIO_CLASS_IDLE)
+def nice_command(command):
+    return ["nice", "-n", "19", "ionice", "-c", "3"] + command
 
 
 class Worker:
@@ -37,7 +33,7 @@ class Worker:
         else:
             command = [__file__]
         create = asyncio.create_subprocess_exec(
-            *command, stdin=asyncio.subprocess.PIPE,
+            *nice_command(command), stdin=asyncio.subprocess.PIPE,
             stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE,
             preexec_fn=os.setsid)
         self.process = await create
@@ -53,7 +49,6 @@ class Worker:
     async def job_runner(self, summary, log, jobs_added_event,
                          appearance_changed_event):
         await self.create_process()
-        _make_process_nicest(self.child_pgid)
         while True:
             await jobs_added_event.wait()
             while True: