- 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::RenderWidgetHost* render_widget_host) {
#if defined(TOOLKIT_VIEWS)
  if (view_->GetDelegate())
    view_->GetDelegate()->DevToolsFocused();
#endif
}
void InspectableWebContentsImpl::ReadyToCommitNavigation(
    content::NavigationHandle* navigation_handle) {
  if (navigation_handle->IsInMainFrame()) {
    if (navigation_handle->GetRenderFrameHost() ==
            GetDevToolsWebContents()->GetMainFrame() &&
        frontend_host_) {
      return;
    }
    frontend_host_ = content::DevToolsFrontendHost::Create(
        web_contents()->GetMainFrame(),
        base::BindRepeating(
            &InspectableWebContentsImpl::HandleMessageFromDevToolsFrontend,
            base::Unretained(this)));
    return;
  }
}
void InspectableWebContentsImpl::DidFinishNavigation(