tray: Support file-drop from OS X Dock

tray: Support file-drop from OS X Dock

A long-standing Apple bug does not call `prepareForDragOperation:sender`
for file drag-and-drop operations from the Dock. So we manually
call our custom `handleDrop:sender` for all drag-and-drop cases (that
is, from the Dock and from Finder).

References to the bug in question:
- http://stackoverflow.com/q/9534543/3309046
- http://openradar.appspot.com/radar?id=1745403

However, we still need to return YES from `prepareForDragOperation:sender`,
otherwise the "drag failed" animation occurs. For the same reason, we also
return YES from `performDragOperation:sender`.