Send empty Ime::Preedit before the Ime::Commit

This should help downstream to automatically clear it.

src/platform_impl/linux/x11/event_processor.rs

@@ -621,6 +621,12 @@ impl<T: 'static> EventProcessor<T> {
                     // If we're composing right now, send the string we've got from X11 via
                     // Ime::Commit.
                     if self.is_composing && keycode == 0 && !written.is_empty() {
+                        let event = Event::WindowEvent {
+                            window_id,
+                            event: WindowEvent::Ime(Ime::Preedit(String::new(), None)),
+                        };
+                        callback(event);
+
                         let event = Event::WindowEvent {
                             window_id,
                             event: WindowEvent::Ime(Ime::Commit(written)),