server: Unmap popup on popup_done event

Seems some applications don't expect their popups to be unmapped without
their consent, so they make act strangely, but at least it doesn't
crash.
Fixes #117

src/lib.rs

@@ -29,6 +29,7 @@ pub trait XConnection: Sized + 'static {
         data: Self::ExtraData,
     );
     fn close_window(&mut self, window: x::Window, data: Self::ExtraData);
+    fn unmap_window(&mut self, window: x::Window);
     fn raise_to_top(&mut self, window: x::Window);
 }