Remove the need for application to set an RPATH.

The WebEngineWidgets module and the WebEngine QtQuick plugin libraries
already have the RPATH set properly in their headers and the application
won't need to link any symbol directly to the Core library.

Remove the RPATH directive for examples and tests and fix the build issue
by making sure that the link directive isn't passed to dependencies
through the prl or pkgconfig file.

Change-Id: Id1f5efb8c9823613e804e8e6356d711d561d72ec
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Add an intermediate Qt5WebEngine module library.

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>
Add a WebEngine.experimental QtQuick plugin library.

Bring this QtWebKit concept to our API as well to contain APIs that
we aren't comfortable adding to the supported-forever group yet and
allow trying them out unofficially for an undetermined number of
releases first.

Change-Id: I52c8655dfd2996ea461ac6c00de975002827a4c3
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Arvid Nilsson <anilsson@blackberry.com>
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>