Add API for intercepting url requests

This patch is adding a QWebEngineUrlRequestInterceptor interface
that can be subclassed to observe or intercept all resource requests
making it possible to implement browser features like adblocking
and setting custom request headers for url requests.
The interceptRequest virtual function is executed on the IO thread.
This patch also includes a unit test for the testable API parts.

Change-Id: Ibe740fc55551a9a5da40794088ccb6d03d913631
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Martin Rotter <rotter.martinos@gmail.com>
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>