Switch to ARC

Opening this so we will have a place to discuss issues we encounter.

  * With the shadow pref enabled, the interface sometimes appears without a shadow. (I'm not 100% sure that the problem is specific to this branch. I just happened to notice it around the same time.)
  * When double-clicking a plug-in, the file isn't moved to the PlugIns folder. You are asked to restart QS if replacing an older plug-in, so it's not ignoring the file completely.

/cc @craigotis