Make the WebEngine QtQuick plubin library a library only that fetches
the API privatly from the official module.

This will allow an experimental plugin library to also have access to
the API classes, which it currently can't since the plugins aren't
deployed the same way as module to <prefix>/lib.

The module currently only export classes privately but the plan is
to make this library the official linking point of entry for
applications along with the Qt5WebEngineWidgets module.
The WebEngineCore library could eventyally be merged into this module
library if we can get gyp to play well with qmake.

Change-Id: I5edb60b412e213b59f791a7b8df9f28c295502de
Reviewed-by: Andras Becsi <andras.becsi@digia.com>