- 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
Emit("drop-files", files);
}
void Tray::OnDropText(const std::string& text) {
  Emit("drop-text", text);
}
void Tray::OnDragEntered() {
  Emit("drag-enter");
}
void Tray::OnDragExited() {
  Emit("drag-leave");
}
void Tray::OnDragEnded() {
  Emit("drag-end");
}
void Tray::SetImage(v8::Isolate* isolate, mate::Handle<NativeImage> image) {
#if defined(OS_WIN)
  tray_icon_->SetImage(image->GetHICON(GetSystemMetrics(SM_CXSMICON)));
#else
  tray_icon_->SetImage(image->image());
#endif
}
void Tray::SetPressedImage(v8::Isolate* isolate,
                           mate::Handle<NativeImage> image) {
#if defined(OS_WIN)
  tray_icon_->SetPressedImage(image->GetHICON(GetSystemMetrics(SM_CXSMICON)));
#else
  tray_icon_->SetPressedImage(image->image());
#endif
}
void Tray::SetToolTip(const std::string& tool_tip) {