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/web/window.rs

@@ -302,6 +302,11 @@ impl Window {
         // Currently a no-op as it does not seem there is good support for this on web
     }
 
+    #[inline]
+    pub fn set_ime_allowed(&self, _allowed: bool) {
+        // Currently not implemented
+    }
+
     #[inline]
     pub fn focus_window(&self) {
         // Currently a no-op as it does not seem there is good support for this on web