**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.h
int GetIndexOfCommandId(int command_id);
  int GetItemCount() const;
  int GetCommandIdAt(int index) const;
  base::string16 GetLabelAt(int index) const;
  base::string16 GetSublabelAt(int index) const;
  base::string16 GetAcceleratorTextAt(int index) const;
  bool IsItemCheckedAt(int index) const;
  bool IsEnabledAt(int index) const;
  bool IsVisibleAt(int index) const;
  bool WorksWhenHiddenAt(int index) const;
  
  base::RepeatingCallback<bool(v8::Local<v8::Value>, int)> is_checked_;
  base::RepeatingCallback<bool(v8::Local<v8::Value>, int)> is_enabled_;
  base::RepeatingCallback<bool(v8::Local<v8::Value>, int)> is_visible_;
  base::RepeatingCallback<bool(v8::Local<v8::Value>, int)> works_when_hidden_;
  base::RepeatingCallback<v8::Local<v8::Value>(v8::Local<v8::Value>, int, bool)>
      get_accelerator_;
  base::RepeatingCallback<bool(v8::Local<v8::Value>, int)>
      should_register_accelerator_;
  base::RepeatingCallback<void(v8::Local<v8::Value>, v8::Local<v8::Value>, int)>
      execute_command_;
  base::RepeatingCallback<void(v8::Local<v8::Value>)> menu_will_show_;
  DISALLOW_COPY_AND_ASSIGN(Menu);
};
}  
}  
namespace mate {
template <>
struct Converter<electron::AtomMenuModel*> {
  static bool FromV8(v8::Isolate* isolate,