Add support for registerProtocolHandler

Extend initialization of URLRequestContextGetterQt to create a
content::ProtocolHandlerRegistry for each content::BrowserContext and add the
registry's URL request interceptor to the front of the interceptor chain.

Implement methods in WebContentsDelegateQt to add/remove protocol handlers
to/from the ProtocolHandlerRegistry.

Add permission request signal and classes for core, quick and widgets.

Add widgets autotest.

Add signal handlers to quicknanobrowser and simplebrowser.

Task-number: QTBUG-62783
Change-Id: I808e7eb9a1cb4d7216686deed4895de14fe46310
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>