- 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
return managed_devtools_web_contents_ && view_->IsDevToolsViewShowing();
}
void InspectableWebContentsImpl::AttachTo(
    scoped_refptr<content::DevToolsAgentHost> host) {
  Detach();
  agent_host_ = std::move(host);
  
  
  agent_host_->AttachClient(this);
}
void InspectableWebContentsImpl::Detach() {
  if (agent_host_)
    agent_host_->DetachClient(this);
  agent_host_ = nullptr;
}
void InspectableWebContentsImpl::Reattach(const DispatchCallback& callback) {
  if (agent_host_) {
    agent_host_->DetachClient(this);
    agent_host_->AttachClient(this);
  }
  callback.Run(nullptr);
}
void InspectableWebContentsImpl::CallClientFunction(