activation_token: Provide GTK 4 helper

Unfortunately, `startup_notify_id` is not nullable so we do the same as GTK.

See https://gitlab.gnome.org/GNOME/gtk/-/commit/6efd1a9dad49b42db778bdc07020dfcf8845e2c8

src/activation_token/gtk4.rs

@@ -0,0 +1,28 @@
+use gtk4::{glib::translate::*, prelude::*};
+
+use crate::ActivationToken;
+
+impl ActivationToken {
+    /// Gets an activation token from a window.
+    ///
+    /// Support for the XDG Activation Protocol was added in GLib 2.76.
+    pub fn from_window(widget: &impl IsA<::gtk4::Widget>) -> Option<Self> {
+        let display = widget.as_ref().display();
+        let context = display.app_launch_context();
+
+        // FIXME Call the vfunc directly since
+        // g_app_launch_context_get_startup_notify_id has NULL checks.
+        unsafe {
+            let klass: *mut gtk4::gio::ffi::GAppLaunchContextClass =
+                std::ptr::addr_of!((*context.class().parent()?)) as *mut _;
+            let get_startup_notify_id = (*klass).get_startup_notify_id.as_ref()?;
+
+            from_glib_full::<_, Option<String>>(get_startup_notify_id(
+                context.as_ptr().cast(),
+                std::ptr::null_mut(),
+                std::ptr::null_mut(),
+            ))
+        }
+        .map(Self::from)
+    }
+}

src/activation_token/mod.rs

@@ -3,6 +3,9 @@ use std::ops::Deref;
 use serde::{Deserialize, Serialize};
 use zbus::zvariant::Type;
 
+#[cfg(any(feature = "gtk4_wayland", feature = "gtk4_x11"))]
+mod gtk4;
+
 /// A token that can be used to activate an application.
 ///
 /// No guarantees are made for the token structure.