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/mod.rs

@@ -154,6 +154,7 @@ mod drop_handler;
 mod event;
 mod event_loop;
 mod icon;
+mod ime;
 mod monitor;
 mod raw_input;
 mod window;