Extend Tray click event with position

- Add a parameter to the `click` event of the  `Tray` API object containing the
coordinates of the tray icon/mouse pointer.
The parameter has `x` and `y` fields for the coordinates.
On Mac OS X the coordinates always refer to the position (lower left corner) of
the tray item (NSStatusItem).
On Aura (Linux, Windows) the coordinates refer to the cursor position (which
would be inside the bounding box of the status icon but with slight differences
depending on where effectively the user clicked).

Usage example:

    tray.on('click', function(pos) {
      // pos.x is x coordinate of the tray icon
      // pos.y is y coordinate of the tray icon
      showCustomTrayMenuAt(pos);
    }

FIX rogerwang/node-webkit#1874
Add manual test for position parameter in Tray click event

- Add a manual test and an example usage (tray only application with a custom
tray menu) of the position parameter in `Tray` `click` event as discussed in
rogerwang/node-webkit#1874
Merge pull request #2799 from mrfabbri/add-tray-click-event-position

Add tray click event position