Ensure that users of declarative that have no need for functionality
provided by the Qt Gui module do not have to link against it.

Any use of QtGui functionality is delegated to providers that can
be installed by another library; QtQuick adds default providers
for this functionality when linked against QtQml.

Task-number: QTBUG-24559
Change-Id: I5e6a58a4198732dc2f8f52f71abfa1152b871aa7
Reviewed-by: Martin Jones <martin.jones@nokia.com>