Introduce WebEngineLibraryInfo for providing paths

This allows us to customize paths on BlackBerry, where QtWebEngine is
currently installed to an different location than the rest of Qt5.

Change-Id: I7144c992bed9fe4d7543ce0d23ee8cd5f5669dd9
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>