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
if (name == "appData")
    return brightray::DIR_APP_DATA;
  else if (name == "userData")
    return brightray::DIR_USER_DATA;
  else if (name == "cache")
    return brightray::DIR_CACHE;
  else if (name == "userCache")
    return brightray::DIR_USER_CACHE;
  else if (name == "home")
    return base::DIR_HOME;
  else if (name == "temp")
    return base::DIR_TEMP;
  else if (name == "userDesktop" || name == "desktop")
    return base::DIR_USER_DESKTOP;
  else if (name == "exe")
    return base::FILE_EXE;
  else if (name == "module")
    return base::FILE_MODULE;