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 +1,48 @@
+use glib::translate::from_glib_full;
+use gtk4::{gio, 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, this
+    /// method will return `None` on older versions.
+    pub fn from_window(widget: &impl IsA<::gtk4::Widget>) -> Option<Self> {
+        if glib::check_version(2, 76, 0).is_some() {
+            #[cfg(feature = "tracing")]
+            tracing::info!("Need glib 2.76 for XDG Activation protocol support");
+
+            return None;
+        }
+
+        let display = widget.as_ref().display();
+        let context = display.app_launch_context();
+
+        // g_app_launch_context_get_startup_notify_id only accepts nullable
+        // parameters since 2.82. On older versions we use the vfunc.
+        if glib::check_version(2, 82, 0).is_some() {
+            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(),
+                ))
+            }
+        } else {
+            unsafe {
+                from_glib_full::<_, Option<String>>(
+                    gio::ffi::g_app_launch_context_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.