extensionSystem: Turn into a class

extensionSystem: Turn into a class

The extension system started out as a set of simple functions, but
gained more state later, and even some hacks to emit signals without
having an object to emit them on.

There is no good reason for that weirdness, so rather than imitating an
object, wrap the existing system into a real ExtensionManager object.

https://bugzilla.gnome.org/show_bug.cgi?id=789852