Remove the placeholder text when IME is activated in text input

- backport iced-rs#2785

Author: kenz-gelsoft

widget/src/text_input.rs

@@ -591,7 +591,13 @@ where
         };
 
         let draw = |renderer: &mut Renderer, viewport| {
-            let paragraph = if text.is_empty() {
+            let paragraph = if text.is_empty()
+                && state
+                    .is_ime_open
+                    .as_ref()
+                    .map(|preedit| preedit.content.is_empty())
+                    .unwrap_or(true)
+            {
                 state.placeholder.raw()
             } else {
                 state.value.raw()