Imjoy plugin loaded check. Or always show some cell output.

When folks first `pip install itkwidgets`, my understanding is folks must refresh the notebook page to load the Imjoy extension. If they don't, `view(thing)` shows no cell output. Should always show some output.

The Env check notebook has some code to cope with that:
```
%%javascript
let needReload = (typeof window.loadImJoyRPC === "undefined");
if (needReload) {
needReload = false;
location.reload();
}
```

Is it possible to put this this check "client" side and show a error message with what to do about it.

Also, the check should check the Imjoy version number, as having an old Imjoy version passes this check, but still get no viewer rendering because error caused by Server/Client Imjoy version incomatablity.