Implement IME-related methods of SctkWinitWindow for apps using single-instance feature

- At least, following apps are using `single-instance` feature:
  - cosmic-launcher
  - cosmic-app-library
- Once libcosmic text widgets supported the IME, these apps start hitting following methods of SctkWinitWindow which is `todo!()` and will crash at startup:
  - `set_ime_cursor_area()`
  - `set_ime_allowed()`
  - `set_ime_purpose()`
- So, this PR implements these method utilizing following wayland protocols.
  - zwp_text_input_v3
  - zwp_text_input_manager_v3

winit/src/platform_specific/wayland/handlers/seat/seat.rs

@@ -8,6 +8,7 @@ use cctk::sctk::{
     seat::{pointer::ThemeSpec, SeatHandler},
 };
 use iced_runtime::keyboard::Modifiers;
+use std::sync::Arc;
 
 impl SeatHandler for SctkState {
     fn seat_state(&mut self) -> &mut cctk::sctk::seat::SeatState {
@@ -141,6 +142,16 @@ impl SeatHandler for SctkState {
             }
             _ => unimplemented!(),
         }
+
+        if let Some(text_input_manager) = self
+            .text_input
+            .is_none()
+            .then_some(self.text_input_manager.as_ref())
+            .flatten()
+        {
+            self.text_input =
+                Some(Arc::new((text_input_manager).get_text_input(&seat, &qh)));
+        }
     }
 
     fn remove_capability(
@@ -193,6 +204,10 @@ impl SeatHandler for SctkState {
             }
             _ => unimplemented!(),
         }
+
+        if let Some(text_input) = self.text_input.take() {
+            text_input.destroy();
+        }
     }
 
     fn remove_seat(