Add ext-data-control-v1 protocol

This is identical to the wlr protocol, and Smithay has implementations
for both.

New clients should use the `ext` protocol where present. Not sure how
widely used it is yet, but we probably should have both for now.

src/wayland/handlers/data_control/ext.rs

@@ -0,0 +1,13 @@
+use crate::state::State;
+use smithay::{
+    delegate_ext_data_control,
+    wayland::selection::ext_data_control::{DataControlHandler, DataControlState},
+};
+
+impl DataControlHandler for State {
+    fn data_control_state(&mut self) -> &mut DataControlState {
+        &mut self.common.ext_data_control_state
+    }
+}
+
+delegate_ext_data_control!(State);

src/wayland/handlers/data_control/mod.rs

@@ -0,0 +1,4 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+mod ext;
+mod wlr;

src/wayland/handlers/data_control/wlr.rs

@@ -8,7 +8,7 @@ use smithay::{
 
 impl DataControlHandler for State {
     fn data_control_state(&mut self) -> &mut DataControlState {
-        &mut self.common.data_control_state
+        &mut self.common.wlr_data_control_state
     }
 }