nautilus extension doesn't build against nautilus 43

The nautilus extension doesn't build against nautilus 43.

nautilus 43 has switched to GTK4 and made several other changes.

To start, you would need to look for libnautilus-extension-4 in configure.ac and simplify the includes in nautilus-extension/nautilus-easytag.c to #include <nautilus-extension.h>. But you'd likely need to make several other changes too.