add CarbonLocalEventMonitor for NSMenu keyboard shortcut observation

[honghaoz]

Not sure why you mentioned the manual way in #1 but didn't include it in the repo, would like to set up a shortcut observation when the menu is open.

This PR:

• adds the CarbonLocalEventMonitor.swift as mentioned in Support menu bar apps that uses NSMenu #1
• adds a convenience method for observing shortcuts.

A example usage is like:

    let localEventMonitor = CarbonLocalEventMonitor.monitor(for: .toggleUnicornMode) {
      // handle the shortcut
    }

    menu.rx.menuOpenState
      .subscribe(onNext: { isOpen in
        if isOpen {
          KeyboardShortcuts.disable(.toggleUnicornMode)
          localEventMonitor.start()
        } else {
          KeyboardShortcuts.enable(.toggleUnicornMode)
          localEventMonitor.stop()
        }
      })
      .disposed(by: disposeBag)

[sindresorhus]

It's easy enough to add the CarbonLocalEventMonitor. I could have done that myself too. The reason I didn't is that I would prefer a proper solution.

As I hinted at in the issue, the proper solution is automatic handling. I imagine we would provide a KeyboardShortcuts.isMenuOpen property, which the user could set to true/false when the menu is open/closed. And under the hood we would handle pausing the global keyboard shortcuts, activating the local monitor, etc.

We could also provide a NSMenu subclass which would handle calling KeyboardShortcuts.isMenuOpen automatically. We still need the property since not everyone can use a custom subclass.

Ideally, we would able to reuse https://github.com/sindresorhus/KeyboardShortcuts/blob/master/Sources/KeyboardShortcuts/CarbonKeyboardShortcuts.swift for local events too. I think it's just about what event specs we provide.

[sindresorhus]

@honghaoz Are you still interested in working on this?

[honghaoz]

Hi, @sindresorhus I'm not working on this one as I'm not an expert on Carbon APIs. I think we should close this PR.