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
Browser::UserTask* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    if (!dict.Get("program", &(out->program)) ||
        !dict.Get("title", &(out->title)))
      return false;
    if (dict.Get("iconPath", &(out->icon_path)) &&
        !dict.Get("iconIndex", &(out->icon_index)))
      return false;
    dict.Get("arguments", &(out->arguments));
    dict.Get("description", &(out->description));
    return true;
  }
};
#endif
}  
namespace atom {
namespace api {
namespace {
int GetPathConstant(const std::string& name) {