- 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_win.cc
if (base::DirectoryExists(full_path))
    return ui::win::OpenFolderViaShell(full_path);
  else
    return ui::win::OpenFileViaShell(full_path);
}
void OpenExternal(const GURL& url,
                  const OpenExternalOptions& options,
                  OpenExternalCallback callback) {
  base::PostTaskAndReplyWithResult(
      base::CreateCOMSTATaskRunner({base::ThreadPool(), base::MayBlock(),
                                    base::TaskPriority::USER_BLOCKING})
          .get(),
      FROM_HERE, base::BindOnce(&OpenExternalOnWorkerThread, url, options),
      std::move(callback));
}
bool MoveItemToTrash(const base::FilePath& path, bool delete_on_fail) {
  base::win::ScopedCOMInitializer com_initializer;
  if (!com_initializer.Succeeded())
    return false;
  Microsoft::WRL::ComPtr<IFileOperation> pfo;
  if (FAILED(::CoCreateInstance(CLSID_FileOperation, nullptr, CLSCTX_ALL,
                                IID_PPV_ARGS(&pfo))))
    return false;
  
  
  if (base::win::GetVersion() >= base::win::Version::WIN8) {
    
    
    if (FAILED(pfo->SetOperationFlags(