Fix unsoundness on Windows (#601)

* Fix unsoundness in windows backend

* Synchronize window state properly

* update changelog and add a comment to execute_in_thread

* Formatting fixes and improve changelog message

src/platform/windows/icon.rs

@@ -22,11 +22,13 @@ pub enum IconType {
     Big = winuser::ICON_BIG as isize,
 }
 
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct WinIcon {
     pub handle: HICON,
 }
 
+unsafe impl Send for WinIcon {}
+
 impl WinIcon {
     #[allow(dead_code)]
     pub fn from_path<P: AsRef<Path>>(path: P) -> Result<Self, util::WinError> {