Add new Ime event for desktop platforms

This commit brings new Ime event to account for preedit state of input
method, also adding `Window::set_ime_allowed` to toggle IME input on
the particular window.

This commit implements API as designed in #1497 for desktop platforms.

Co-authored-by: Artur Kovacs <kovacs.artur.barnabas@gmail.com>
Co-authored-by: Markus Siglreithmaier <m.siglreith@gmail.com>
Co-authored-by: Murarth <murarth@gmail.com>
Co-authored-by: Yusuke Kominami <yukke.konan@gmail.com>
Co-authored-by: moko256 <koutaro.mo@gmail.com>

src/platform_impl/windows/window_state.rs

@@ -42,6 +42,9 @@ pub struct WindowState {
     pub preferred_theme: Option<Theme>,
     pub high_surrogate: Option<u16>,
     pub window_flags: WindowFlags,
+
+    pub ime_state: ImeState,
+    pub ime_allowed: bool,
 }
 
 #[derive(Clone)]
@@ -101,6 +104,13 @@ bitflags! {
     }
 }
 
+#[derive(Eq, PartialEq)]
+pub enum ImeState {
+    Disabled,
+    Enabled,
+    Preedit,
+}
+
 impl WindowState {
     pub fn new(
         attributes: &WindowAttributes,
@@ -132,6 +142,9 @@ impl WindowState {
             preferred_theme,
             high_surrogate: None,
             window_flags: WindowFlags::empty(),
+
+            ime_state: ImeState::Disabled,
+            ime_allowed: false,
         }
     }