Add Window::on_present_notify to ack about drawing

That's a way to communicate to winit that you'll present to the window.
While it's a no-op for now, it'll be used to throttle drawing.

src/platform_impl/linux/x11/window.rs

@@ -1731,6 +1731,11 @@ impl UnownedWindow {
             .unwrap();
     }
 
+    #[inline]
+    pub fn pre_present_notify(&self) {
+        // TODO timer
+    }
+
     #[inline]
     pub fn raw_window_handle(&self) -> RawWindowHandle {
         let mut window_handle = XlibWindowHandle::empty();