[Fix] time.clock() deprecated in Python 3.8

[marco97pa]

Replaced all calls to time.clock() with time.perf_counter().

time.clock() has been removed from the time module in Python 3.8.

This PR closes #108 and makes the app executable again on Python 3.8+

Tested on my Windows machine with Python 3.8

Before the fix, the app was crashing unexpectedly with the following Traceback:

  File "main.py", line 902, in <module>
    main()
  File "main.py", line 898, in main
    pyglet.app.run()
  File "C:\Users\***\AppData\Roaming\Python\Python38\site-packages\pyglet\app\__init__.py", line 107, in run
    event_loop.run()
  File "C:\Users\***\AppData\Roaming\Python\Python38\site-packages\pyglet\app\base.py", line 167, in run
    timeout = self.idle()
  File "C:\Users\***\AppData\Roaming\Python\Python38\site-packages\pyglet\app\base.py", line 237, in idle
    redraw_all = self.clock.call_scheduled_functions(dt)
  File "C:\Users\***\AppData\Roaming\Python\Python38\site-packages\pyglet\clock.py", line 292, in call_scheduled_functions
    item.func(now - item.last_ts, *item.args, **item.kwargs)
  File "main.py", line 571, in update
    self.model.process_queue()
  File "main.py", line 422, in process_queue
    start = time.clock()