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/window.rs

@@ -1564,3 +1564,17 @@ impl Default for ImePurpose {
         Self::Normal
     }
 }
+
+/// An opaque token used to activate the [`Window`].
+///
+/// [`Window`]: crate::window::Window
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct ActivationToken {
+    pub(crate) _token: String,
+}
+
+impl ActivationToken {
+    pub(crate) fn _new(_token: String) -> Self {
+        Self { _token }
+    }
+}