feat(positioner, window-state): impl WindowExt for WebviewWindow (#1283)

closes #1281

plugins/window-state/src/cmd.rs

@@ -26,8 +26,6 @@ pub async fn restore_state<R: Runtime>(
         .ok_or_else(|| format!("Invalid state flags bits: {}", flags))?;
     app.get_webview_window(&label)
         .ok_or_else(|| format!("Couldn't find window with label: {}", label))?
-        .as_ref()
-        .window()
         .restore_state(flags)
         .map_err(|e| e.to_string())?;
     Ok(())