- Electron version:1.3.3
- Operating system:win10/mac

the Tray instance events shold provide the tray hover event. 
`tray.setToolTip` has limitations when do more operation.

atom/browser/api/atom_api_tray.cc
mate::Arguments* args) {
  if (!Browser::Get()->is_ready()) {
    args->ThrowError("Cannot create Tray before app is ready");
    return nullptr;
  }
  return new Tray(args->isolate(), args->GetThis(), image);
}
void Tray::OnClicked(const gfx::Rect& bounds, int modifiers) {
  EmitWithFlags("click", modifiers, bounds);
}
void Tray::OnDoubleClicked(const gfx::Rect& bounds, int modifiers) {
  EmitWithFlags("double-click", modifiers, bounds);
}
void Tray::OnRightClicked(const gfx::Rect& bounds, int modifiers) {
  EmitWithFlags("right-click", modifiers, bounds);
}
void Tray::OnBalloonShow() {
  Emit("balloon-show");
}
void Tray::OnBalloonClicked() {
  Emit("balloon-click");
}
void Tray::OnBalloonClosed() {
  Emit("balloon-closed");
}
void Tray::OnDrop() {
  Emit("drop");
}
void Tray::OnDropFiles(const std::vector<std::string>& files) {