Improve QWebEngineUrlScheme API

Following feedback from 5.12 API review:

- Use enum class for Syntax
- Add Q_FLAG for Flags
- Mark constructor from name as explicit
- Rename Secure to SecureScheme
- Rename Local to LocalScheme
- Rename addScheme to registerScheme
- Rename findScheme to schemeByName

Task-number: QTBUG-70247
Change-Id: Iae332c8d9843349506e8a4b07d70f0d234597375
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>