shell.xdg: add XdgRequest::AckConfigure

This will be used for convenient resize state tracking.

src/wayland/shell/xdg/mod.rs

@@ -941,4 +941,11 @@ pub enum XdgRequest<R> {
         /// location of the menu request
         location: (i32, i32),
     },
+    /// A surface has acknowledged a configure serial.
+    AckConfigure {
+        /// The surface.
+        surface: wl_surface::WlSurface,
+        /// The configure serial.
+        serial: u32,
+    },
 }

src/wayland/shell/xdg/xdg_handlers.rs

@@ -333,6 +333,12 @@ where
                     role_data.configured = true;
                 })
                 .expect("xdg_surface exists but surface has not shell_surface role?!");
+
+            let mut user_impl = data.shell_data.user_impl.borrow_mut();
+            (&mut *user_impl)(XdgRequest::AckConfigure {
+                surface: data.wl_surface.clone(),
+                serial,
+            });
         }
         _ => unreachable!(),
     }

src/wayland/shell/xdg/zxdgv6_handlers.rs

@@ -349,6 +349,12 @@ fn xdg_surface_implementation<R>(
                     role_data.configured = true;
                 })
                 .expect("xdg_surface exists but surface has not shell_surface role?!");
+
+            let mut user_impl = data.shell_data.user_impl.borrow_mut();
+            (&mut *user_impl)(XdgRequest::AckConfigure {
+                surface: data.wl_surface.clone(),
+                serial,
+            });
         }
         _ => unreachable!(),
     }