Implement HoveredFile and HoveredFileCancelled on Windows (#662)

* Implement HoveredFile and HoveredFileCancelled on Windows (#448)

* Update CHANGELOG.

* Applied code organizational corrections and fixed IDropHandler leak on window destroy.

* Moved FileDropHandle to a separate file.

src/platform/windows/mod.rs

@@ -49,6 +49,7 @@ unsafe impl Send for WindowId {}
 unsafe impl Sync for WindowId {}
 
 mod dpi;
+mod drop_handler;
 mod event;
 mod events_loop;
 mod icon;