Custom playdata exporters cause widget engine installation to fail

This appears to be because the widget installer is attempting to serialize a `dict_keys` object, which is not possible:

```
manifest_data["meta_data"]["playdata_exporters"] = exporter_mappings.keys()
```

The correction appears to be to simply convert the `dict_keys` to a list:

```
manifest_data["meta_data"]["playdata_exporters"] = list(
    exporter_mappings.keys()
)
```
