The new `setAsDefaultProtocolClient`  and `removeAsDefaultProtocolClient` APIs are great, but without a way to query the current state, it's not possible to implement a UI element like this one. I think it'd be great to add `isDefaultProtocolClient`!

![image](https://cloud.githubusercontent.com/assets/1037212/14545184/fc1a6fac-026a-11e6-98d5-8bf6b2f9678e.png)

Also, I find the naming of these APIs pretty confusing. On Mac OS X and iOS `mailto` is referred to as a "URL Scheme", not a protocol (https://developer.apple.com/library/ios/featuredarticles/iPhoneURLScheme_Reference), and it seems like that might be the case on Windows too. (https://msdn.microsoft.com/en-us/library/aa767914(v=vs.85).aspx)

It'd be nice to clarify that this is registering the app system-wide, while the other `protocol` module is only for handling requests initiated from within the app. 

Maybe we could change these to `registerForURLScheme`, `isRegisteredForURLScheme`?

atom/browser/api/atom_api_app.cc
v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  bool prevent_default = Emit(
      "login",
      WebContents::CreateFrom(isolate(), login_handler->GetWebContents()),
      login_handler->request(),
      login_handler->auth_info(),
      base::Bind(&PassLoginInformation, make_scoped_refptr(login_handler)));
  
  if (!prevent_default)
    login_handler->CancelAuth();
}
void App::OnCreateWindow(const GURL& target_url,
                         const std::string& frame_name,
                         WindowOpenDisposition disposition,
                         int render_process_id,
                         int render_frame_id) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  content::RenderFrameHost* rfh =
      content::RenderFrameHost::FromID(render_process_id, render_frame_id);
  content::WebContents* web_contents =
      content::WebContents::FromRenderFrameHost(rfh);
  if (web_contents) {
    auto api_web_contents = WebContents::CreateFrom(isolate(), web_contents);
    api_web_contents->OnCreateWindow(target_url, frame_name, disposition);
  }
}
void App::AllowCertificateError(