Clean up icon distribution code.

* Renamed "nautilus_mix" to simply "mix".

* Fixed a silly math error (taking the cube root of 0.9 and then cubing
  it right back).

* Clarify what kind of jitter is being prevented by subtracting 1.0
  from the grid width.
1 job for eel-distribute-items in 4 minutes and 14 seconds (queued for 2 seconds)