Implement windows focus key press/release on Windows (#1307)

* X11: Sync key press/release with window focus

* When a window loses focus, key release events are issued for all pressed keys
* When a window gains focus, key press events are issued for all pressed keys
* Adds `is_synthetic` field to `WindowEvent` variant `KeyboardInput`
  to indicate that these events are synthetic.
* Adds `is_synthetic: false` to `WindowEvent::KeyboardInput` events issued
  on all other platforms

* Implement windows focus key press/release on Windows

* Docs

Co-authored-by: Murarth <murarth@gmail.com>

src/platform_impl/windows/event.rs

@@ -27,6 +27,16 @@ pub fn get_key_mods() -> ModifiersState {
     mods
 }
 
+pub fn get_pressed_keys() -> impl Iterator<Item = c_int> {
+    let mut keyboard_state = vec![0u8; 256];
+    unsafe { winuser::GetKeyboardState(keyboard_state.as_mut_ptr()) };
+    keyboard_state
+        .into_iter()
+        .enumerate()
+        .filter(|(_, p)| (*p & (1 << 7)) != 0) // whether or not a key is pressed is communicated via the high-order bit
+        .map(|(i, _)| i as c_int)
+}
+
 unsafe fn get_char(keyboard_state: &[u8; 256], v_key: u32, hkl: HKL) -> Option<char> {
     let mut unicode_bytes = [0u16; 5];
     let len = winuser::ToUnicodeEx(