- 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
const base::string16& title,
                              const base::string16& contents) {
}
void TrayIcon::PopUpContextMenu(const gfx::Point& pos,
                                AtomMenuModel* menu_model) {
}
gfx::Rect TrayIcon::GetBounds() {
  return gfx::Rect();
}
void TrayIcon::NotifyClicked(const gfx::Rect& bounds, int modifiers) {
  for (TrayIconObserver& observer : observers_)
    observer.OnClicked(bounds, modifiers);
}
void TrayIcon::NotifyDoubleClicked(const gfx::Rect& bounds, int modifiers) {
  for (TrayIconObserver& observer : observers_)
    observer.OnDoubleClicked(bounds, modifiers);
}
void TrayIcon::NotifyBalloonShow() {
  for (TrayIconObserver& observer : observers_)
    observer.OnBalloonShow();
}
void TrayIcon::NotifyBalloonClicked() {
  for (TrayIconObserver& observer : observers_)
    observer.OnBalloonClicked();
}
void TrayIcon::NotifyBalloonClosed() {
  for (TrayIconObserver& observer : observers_)
    observer.OnBalloonClosed();
}
void TrayIcon::NotifyRightClicked(const gfx::Rect& bounds, int modifiers) {