extensionPrefs: Load D-Bus interface from own resource

extensionPrefs: Load D-Bus interface from own resource

Using the "regular" loadInterfaceXML() helper means less code duplication,
but it also ties us to the resource used by gnome-shell.

In order to untangle the extension app from core gnome-shell, change that
to load the interface from the existing data resource instead. While that
does involve reimplementing loadInterfaceXML(), it's not too bad actually
with the resource-loading code stripped (as the data resource is already
loaded by the package module).

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1081