- 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
g_web_contents_instances_.remove(this);
  
  if (GetDevToolsWebContents()) {
    if (managed_devtools_web_contents_)
      managed_devtools_web_contents_->SetDelegate(nullptr);
    
    
    WebContentsDestroyed();
  }
  
}
InspectableWebContentsView* InspectableWebContentsImpl::GetView() const {
  return view_.get();
}
content::WebContents* InspectableWebContentsImpl::GetWebContents() const {
  return web_contents_.get();
}
content::WebContents* InspectableWebContentsImpl::GetDevToolsWebContents()
    const {
  if (external_devtools_web_contents_)
    return external_devtools_web_contents_;
  else
    return managed_devtools_web_contents_.get();
}
void InspectableWebContentsImpl::InspectElement(int x, int y) {