Add platform::startup_notify for Wayland/X11

The utils in this module should help the users to activate the windows
they create, as well as manage activation tokens environment variables.

The API is essential for Wayland in the first place, since some
compositors may decide initial focus of the window based on whether
the activation token was during the window creation.

Fixes #2279.

Co-authored-by: John Nunley <jtnunley01@gmail.com>

src/platform_impl/linux/wayland/window/mod.rs

@@ -22,6 +22,7 @@ use sctk::shell::WaylandSurface;
 use crate::dpi::{LogicalSize, PhysicalPosition, PhysicalSize, Position, Size};
 use crate::error::{ExternalError, NotSupportedError, OsError as RootOsError};
 use crate::event::{Ime, WindowEvent};
+use crate::event_loop::AsyncRequestSerial;
 use crate::platform_impl::{
     Fullscreen, MonitorHandle as PlatformMonitorHandle, OsError,
     PlatformSpecificWindowBuilderAttributes as PlatformAttributes,
@@ -168,6 +169,14 @@ impl Window {
             _ => (),
         };
 
+        // Activate the window when the token is passed.
+        if let (Some(xdg_activation), Some(token)) = (
+            xdg_activation.as_ref(),
+            platform_attributes.activation_token,
+        ) {
+            xdg_activation.activate(token._token, &surface);
+        }
+
         // XXX Do initial commit.
         window.commit();
 
@@ -496,13 +505,31 @@ impl Window {
 
         self.attention_requested.store(true, Ordering::Relaxed);
         let surface = self.surface().clone();
-        let data =
-            XdgActivationTokenData::new(surface.clone(), Arc::downgrade(&self.attention_requested));
+        let data = XdgActivationTokenData::Attention((
+            surface.clone(),
+            Arc::downgrade(&self.attention_requested),
+        ));
         let xdg_activation_token = xdg_activation.get_activation_token(&self.queue_handle, data);
         xdg_activation_token.set_surface(&surface);
         xdg_activation_token.commit();
     }
 
+    pub fn request_activation_token(&self) -> Result<AsyncRequestSerial, NotSupportedError> {
+        let xdg_activation = match self.xdg_activation.as_ref() {
+            Some(xdg_activation) => xdg_activation,
+            None => return Err(NotSupportedError::new()),
+        };
+
+        let serial = AsyncRequestSerial::get();
+
+        let data = XdgActivationTokenData::Obtain((self.window_id, serial));
+        let xdg_activation_token = xdg_activation.get_activation_token(&self.queue_handle, data);
+        xdg_activation_token.set_surface(self.surface());
+        xdg_activation_token.commit();
+
+        Ok(serial)
+    }
+
     #[inline]
     pub fn set_cursor_grab(&self, mode: CursorGrabMode) -> Result<(), ExternalError> {
         self.window_state.lock().unwrap().set_cursor_grab(mode)