Add methods to issue various types of HTTP requests

Added class QWebEngineHttpRequest, which describes a
GET or POST HTTP Request. Also added overloads of method
"load" to QWebEngineView, QWebEnginePage and WebContentsAdapter,
which issue such a request.
These can be used for example to simulate form-submissions.

Task-number: QTBUG-53314
Task-number: QTBUG-53372
Change-Id: I85ac8cdd3d1557905b35e3172b922aba356d1c41
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>