Merge lp://staging/~ted/url-dispatcher/is-scope-check into lp://staging/url-dispatcher/16.10

Scope testing for the scope:// URL and overlays that should be over the dash

This MR adds in the ability to check to see if something is a scope, which we use for a couple of things. The first is so that we can return errors to the calling process for the scope:// URL early, meaning applications can know if their call resulted in an error without the user just being confused why the button didn't work. The second is for overlays where we get called by a scope, so we need to overlay the window on top of the dash instead of the scope process.

I'll note that the Unity Scopes API makes testing its usage basically impossible. There is some pretty huge hoops that are jumped through here to test it. Not happy about them, but the code is tested. Disappointed in the scopes API.