Undone the previous change, used and official example and spli the coe to multiple files

src/provide_registry_state.rs

@@ -0,0 +1,15 @@
+use smithay_client_toolkit::{
+    output::OutputState,
+    registry::{ProvidesRegistryState, RegistryState},
+    registry_handlers,
+    seat::SeatState,
+};
+
+use crate::simple_layer::SimpleLayer;
+
+impl ProvidesRegistryState for SimpleLayer {
+    fn registry(&mut self) -> &mut RegistryState {
+        &mut self.registry_state
+    }
+    registry_handlers![OutputState, SeatState];
+}