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/event_loop.rs

@@ -9,7 +9,7 @@
 //! handle events.
 use std::marker::PhantomData;
 use std::ops::Deref;
-use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
 use std::{error, fmt};
 
 use raw_window_handle::{HasRawDisplayHandle, RawDisplayHandle};
@@ -437,3 +437,29 @@ pub enum DeviceEvents {
     /// Never capture device events.
     Never,
 }
+
+/// A unique identifier of the winit's async request.
+///
+/// This could be used to identify the async request once it's done
+/// and a specific action must be taken.
+///
+/// One of the handling scenarious could be to maintain a working list
+/// containing [`AsyncRequestSerial`] and some closure associated with it.
+/// Then once event is arriving the working list is being traversed and a job
+/// executed and removed from the list.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct AsyncRequestSerial {
+    serial: u64,
+}
+
+impl AsyncRequestSerial {
+    // TODO(kchibisov) remove `cfg` when the clipboard will be added.
+    #[allow(dead_code)]
+    pub(crate) fn get() -> Self {
+        static CURRENT_SERIAL: AtomicU64 = AtomicU64::new(0);
+        // NOTE: we rely on wrap around here, while the user may just request
+        // in the loop u64::MAX times that's issue is considered on them.
+        let serial = CURRENT_SERIAL.fetch_add(1, Ordering::Relaxed);
+        Self { serial }
+    }
+}