Add widget based spellchecker example

This example demonstrates how to convert 'dic' dictionary files
into 'bdict' binary format using qwebengine_convert_dict tool.
It shows how to implement language selection. It adds two dummy
dictionaries.

Change-Id: Iffc23a0ed4e51cbc749f666c8f565fafb3739a9b
Reviewed-by: Michael Brüning <michael.bruning@qt.io>