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* Electron version: 1.6.2
* Operating system: macOS

### Expected behavior
Allow users to register a callback for menu open and close events.
```javascript
const menu = new Menu()

menu.onOpen(() => { /* Do something when the menu opens */ })
menu.onClose(() => { /* Do something when the menu closes */ })
```
<!-- What do you think should happen? -->

### Actual behavior

Not supported


NSMenuDelegate exposes these events: https://developer.apple.com/reference/appkit/nsmenudelegate

Not sure about Windows or Linux

atom/browser/ui/atom_menu_model.cc
#include "atom/browser/ui/atom_menu_model.h"
#include "base/stl_util.h"
namespace atom {
AtomMenuModel::AtomMenuModel(Delegate* delegate)
    : ui::SimpleMenuModel(delegate),
      delegate_(delegate) {
}
AtomMenuModel::~AtomMenuModel() {
}
void AtomMenuModel::SetRole(int index, const base::string16& role) {
  int command_id = GetCommandIdAt(index);
  roles_[command_id] = role;
}
base::string16 AtomMenuModel::GetRoleAt(int index) {