I'm trying out a new scope strategy. The CloseButton exists as a real
struct with the `Component` trait. There's a bundle spawning utility
function and several observer systems. The observer systems have been
added directly to the app, which I *believe* means they won't be
duplicated the way the Big Red Button's observers will.
Fixed struct field visibility, initialize the resource, and register it
with the EGUI Debug overlay. The button observers have also been updated
to use the resource.
