api: add ActivationToken::{from,into}_raw

This is needed when passing and getting token from the IPC to activate
the window.

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

@@ -165,7 +165,7 @@ impl Window {
         if let (Some(xdg_activation), Some(token)) =
             (xdg_activation.as_ref(), attributes.platform_specific.activation_token)
         {
-            xdg_activation.activate(token._token, &surface);
+            xdg_activation.activate(token.token, &surface);
         }
 
         // XXX Do initial commit.