- Electron version: 1.3.6
- Operating system: Windows 7 x64
- Other: Microsoft Office 2013 (will make more sense in a bit)

`shell.openItem()` is a blocking call, it would be nice if it's async or has an async version to handle synchronous external application behavior.

Reproduce: 
Open a password-protected Microsoft Excel file, `shell.openItem()` will be blocking until the Password window is submitted or dismissed. Consequently Microsoft Word does not have this issue.

Related to #2761.

shell/common/platform_util.h
namespace platform_util {
typedef base::OnceCallback<void(const std::string&)> OpenExternalCallback;
void ShowItemInFolder(const base::FilePath& full_path);
bool OpenItem(const base::FilePath& full_path);
struct OpenExternalOptions {
  bool activate = true;
  base::FilePath working_dir;
};
void OpenExternal(const GURL& url,
                  const OpenExternalOptions& options,
                  OpenExternalCallback callback);
bool MoveItemToTrash(const base::FilePath& full_path, bool delete_on_fail);
void Beep();
#if defined(OS_MACOSX)
bool GetLoginItemEnabled();
bool SetLoginItemEnabled(bool enabled);
#endif
#if defined(OS_LINUX)
bool GetDesktopName(std::string* setme);
#endif
}  
#endif