Move QWebEngineCallback to core API and templatize CallbackDirectory

This undocumented helper template will also be utilized by the
core API since Chromium's content API heavily relies on callbacks.
To make the internal CallbackDirectory easily extendable with new
types, templatize its callback registration and dispatching functions
to be able to replace its type switches and the internal union.

Change-Id: I3f636fef48973ac95253f1c1bd396550286e571e
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>