On X11, store window target on EventProcessor

Remove the redundant `Rc` to access the window target.

src/platform_impl/linux/x11/window.rs

@@ -123,7 +123,7 @@ impl SharedState {
 unsafe impl Send for UnownedWindow {}
 unsafe impl Sync for UnownedWindow {}
 
-pub(crate) struct UnownedWindow {
+pub struct UnownedWindow {
     pub(crate) xconn: Arc<XConnection>, // never changes
     xwindow: xproto::Window,            // never changes
     #[allow(dead_code)]