- 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/ui/tray_icon.cc
for (TrayIconObserver& observer : observers_)
    observer.OnRightClicked(bounds, modifiers);
}
void TrayIcon::NotifyDrop() {
  for (TrayIconObserver& observer : observers_)
    observer.OnDrop();
}
void TrayIcon::NotifyDropFiles(const std::vector<std::string>& files) {
  for (TrayIconObserver& observer : observers_)
    observer.OnDropFiles(files);
}
void TrayIcon::NotifyDropText(const std::string& text) {
  for (TrayIconObserver& observer : observers_)
    observer.OnDropText(text);
}
void TrayIcon::NotifyDragEntered() {
  for (TrayIconObserver& observer : observers_)
    observer.OnDragEntered();
}
void TrayIcon::NotifyDragExited() {
  for (TrayIconObserver& observer : observers_)
    observer.OnDragExited();
}
void TrayIcon::NotifyDragEnded() {
  for (TrayIconObserver& observer : observers_)
    observer.OnDragEnded();
}
}