**Is your feature request related to a problem? Please describe.**

MenuItem labels are meant to be short. There's currently no place to elaborate/clarify them.

**Describe the solution you'd like**

`NSMenuItem` supports a [`toolTip`](https://developer.apple.com/documentation/appkit/nsmenuitem/1514848-tooltip?language=objc) property. Would be nice if Electron could expose this.

From what I could find, [GTK (Linux) does not have any way to set tooltip on menu items](https://developer.gnome.org/gtk3/stable/GtkMenuItem.html).

No idea about Windows.

**Describe alternatives you've considered**

None.

**Additional context**

No.
shell/browser/api/atom_api_menu.cc
v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  return should_register_accelerator_.Run(GetWrapper(), command_id);
}
void Menu::ExecuteCommand(int command_id, int flags) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  execute_command_.Run(GetWrapper(),
                       mate::internal::CreateEventFromFlags(isolate(), flags),
                       command_id);
}
void Menu::OnMenuWillShow(ui::SimpleMenuModel* source) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  menu_will_show_.Run(GetWrapper());
}
void Menu::InsertItemAt(int index,
                        int command_id,
                        const base::string16& label) {
  model_->InsertItemAt(index, command_id, label);
}
void Menu::InsertSeparatorAt(int index) {
  model_->InsertSeparatorAt(index, ui::NORMAL_SEPARATOR);
}
void Menu::InsertCheckItemAt(int index,