Add OS X proxy icon to title bar

Fixes #1891.

Test Plan:

  * Opened Atom window in directory, verified directory icon shows up
  * Opened a file and verified icon changed
  * Right clicked icon, verified menu appears
  * Drag file to terminal and it pastes the file path (yay)
  * Opened a new Atom window and opened some files in that to make sure that
    it didn't change the original window

I didn't add any specs for this - advice welcome here. I also haven't tested
on Windows or Linux but it looks like `setRepresentedFilename` is a noop on
those platforms.
path -> proxyIconPath

Test plan:
Crossed fingers
Implement setDocumentEdited too

As requested by @philipgiuliani

Test Plan:
Opened an existing file and made a modification. The icon fades at the same
time the tab close button changes to a circle. Undo makes the icon opaque
again.
Don't use project path as fallback icon for item with no path

Stops the project folder icon appearing in places that don't make sense
like settings and new files.

Test Plan:

  * Opened settings and didn't see the project folder icon
  * Closed all tabs and saw the project folder icon
Merge pull request #3171 from dmnd/proxy-icon

Add OS X proxy icon to title bar