- 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/browser/ui/inspectable_web_contents_impl.cc
content::WebContents* devtools) {
  if (!managed_devtools_web_contents_)
    external_devtools_web_contents_ = devtools;
}
void InspectableWebContentsImpl::ShowDevTools(bool activate) {
  if (embedder_message_dispatcher_) {
    if (managed_devtools_web_contents_)
      view_->ShowDevTools(activate);
    return;
  }
  activate_ = activate;
  
  
  embedder_message_dispatcher_ =
      DevToolsEmbedderMessageDispatcher::CreateForDevToolsFrontend(this);
  if (!external_devtools_web_contents_) {  
    managed_devtools_web_contents_ = content::WebContents::Create(
        content::WebContents::CreateParams(web_contents_->GetBrowserContext()));
    managed_devtools_web_contents_->SetDelegate(this);
  }
  Observe(GetDevToolsWebContents());
  AttachTo(content::DevToolsAgentHost::GetOrCreateFor(web_contents_.get()));
  GetDevToolsWebContents()->GetController().LoadURL(
      GetDevToolsURL(can_dock_), content::Referrer(),
      ui::PAGE_TRANSITION_AUTO_TOPLEVEL, std::string());
}
void InspectableWebContentsImpl::CloseDevTools() {