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
bool succeed = false;
  base::FilePath path;
  int key = GetPathConstant(name);
  if (key >= 0)
    succeed = PathService::Get(key, &path);
  if (!succeed)
    args->ThrowError("Failed to get path");
  return path;
}
void App::SetPath(mate::Arguments* args,
                  const std::string& name,
                  const base::FilePath& path) {
  if (!path.IsAbsolute()) {
    args->ThrowError("path must be absolute");
    return;
  }
  bool succeed = false;
  int key = GetPathConstant(name);
  if (key >= 0)
    succeed = PathService::OverrideAndCreateIfNeeded(key, path, true, false);
  if (!succeed)
    args->ThrowError("Failed to set path");
}
void App::SetDesktopName(const std::string& desktop_name) {
#if defined(OS_LINUX)
  scoped_ptr<base::Environment> env(base::Environment::Create());
  env->SetVar("CHROME_DESKTOP", desktop_name);
#endif
}
void App::AllowNTLMCredentialsForAllDomains(bool should_allow) {