The Extensions app code is now independent enough from the rest of the code
base to move it to its own subprojects, like we did for the extensions-tool.
This allows for stand-alone builds of the app, which we are about to use
for distributing it as flatpak.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133
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
