- 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
#include "shell/common/platform_util.h"
#include <windows.h>  
#include <atlbase.h>
#include <comdef.h>
#include <commdlg.h>
#include <dwmapi.h>
#include <objbase.h>
#include <shellapi.h>
#include <shlobj.h>
#include <wrl/client.h>
#include "base/bind.h"
#include "base/bind_helpers.h"
#include "base/files/file_path.h"
#include "base/files/file_util.h"
#include "base/logging.h"
#include "base/stl_util.h"
#include "base/strings/string_util.h"
#include "base/strings/utf_string_conversions.h"
#include "base/task/post_task.h"
#include "base/threading/scoped_blocking_call.h"
#include "base/win/registry.h"
#include "base/win/scoped_co_mem.h"
#include "base/win/scoped_com_initializer.h"
#include "base/win/windows_version.h"
#include "content/public/browser/browser_task_traits.h"
#include "ui/base/win/shell.h"
#include "url/gurl.h"
namespace {
class DeleteFileProgressSink : public IFileOperationProgressSink {