- 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
atom::TrayIcon::HighlightMode* out) {
    std::string mode;
    if (ConvertFromV8(isolate, val, &mode)) {
      if (mode == "always") {
        *out = atom::TrayIcon::HighlightMode::ALWAYS;
        return true;
      }
      if (mode == "selection") {
        *out = atom::TrayIcon::HighlightMode::SELECTION;
        return true;
      }
      if (mode == "never") {
        *out = atom::TrayIcon::HighlightMode::NEVER;
        return true;
      }