**Is your feature request related to a problem? Please describe.**
The following cause a bad effect with the window appearing/exploding (glitch?) from top-left :
```
//user clicks on tray-icon => context-menu => show and I'm supposed to pop-up the app window
app.dock.show();
window.show();
```
This happens because app.dock.show() is not synchronous thus genie effect from the dockbar can't take place because the dock icon is not yet created when we call window.show().

**Describe the solution you'd like**
app.dock.show() should return a promise fullfilled when the icon is in the dock.

**Describe alternatives you've considered**
Right now I'm doing this and it works:
```
app.dock.show();
app.dock.setIcon(appIcon);
setTimeout(window.show, 500);
```

Another is to provide a way to disable system animation (OSX) for window.show() / window.hide().

**Additional context**
Of course I have no idea about internal API availability/functionality. If not possible we can just close this issue and other people will have a reference about it! :)
atom/browser/browser.h
void OpenURL(const std::string& url);
#if defined(OS_MACOSX)
  
  void NewWindowForTab();
#endif  
  
  
  void Activate(bool has_visible_windows);
  
  void WillFinishLaunching();
  void DidFinishLaunching(const base::DictionaryValue& launch_info);
  void OnAccessibilitySupportChanged();
  
  void RequestLogin(scoped_refptr<LoginHandler> login_handler,
                    std::unique_ptr<base::DictionaryValue> request_details);
  void PreMainMessageLoopRun();
  
  
  void SetMainMessageLoopQuitClosure(base::OnceClosure quit_closure);
  void AddObserver(BrowserObserver* obs) { observers_.AddObserver(obs); }
  void RemoveObserver(BrowserObserver* obs) { observers_.RemoveObserver(obs); }
  bool is_shutting_down() const { return is_shutdown_; }
  bool is_quiting() const { return is_quiting_; }
  bool is_ready() const { return is_ready_; }
  util::Promise* WhenReady(v8::Isolate* isolate);
 protected:
  
  std::string GetExecutableFileVersion() const;