Add public QtWebEngineCore C++ API

This patch introduces a new public C++ API layer in preparation to make
it possible to integrate with lower level Chromium features related mostly
to networking operations like accessing and blocking cookies, custom
request headers, etc.
This API layer can be used both by Qt Widgets and Qt Quick applications
with a small C++ core. It should contatain API to access features that
usually run on the IO thread to make it possible to perform heavy tasks
that would otherwise require costly context switches to the UI thread.
Furthermore for these features a QML API does either not make sense,
since they are non-UI-related, or a QML API is simply not feasible,
because the API is meant for advanced usecases like web browser
development (i.e. custom protocol handlers, network traffic
interception cookie syncing, etc.).
In the long term this layer could also make it possible to reduce code
duplication in the widgets and quick layers by moving common parts to
the core layer.
The new API is built entirely by qmake as a separate static library which
is then linked into the QtWebEngineCore library built by gyp and ninja,
to prevent the build options passed to Chromium to break the API layer.
As a first step this only contains the global headers for core.

Change-Id: Iccf8544587cde7c0d9c6abd462e4766bf9ec81ae
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>