Provide the different mime types in QMimeData via custom_data for a QWebEnginePage

Provide the different mime types in QMimeData via custom_data for a QWebEnginePage. If they can be converted to be shown as strings then this will suffice in that respect. Otherwise if something is a known type then it can be converted in the expected manner.

DataTransfer.setData() / DataTransfer.getData() works fine when drag n dropping within the webengineview.

The use case is a hybrid application that transfers custom data via drag n drop from and to the webengineview.