extensionPrefs: Attach extension object to each row

extensionPrefs: Attach extension object to each row

Each row represents an extension, so it makes sense to associate the
rows with the actual extensions instead of linking rows and extensions
by looking up the UUID in the external extensions map in ExtensionUtils.

This will also make it much easier to stop using the shared extension
loading / map in favor of the extension D-Bus API.

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