doc: Control.qml and Style.qml must have documentation

Control.qml and Style.qml must be known to qdoc, so they must have
minimal documentation to tell qdoc to ignore them because they are
internal. They must also be listed in the sources variable in the
.qdocconf file.

Task-number: QTBUG-29777
Change-Id: If601e8a52a7f1685004c03b471d2e0007fbdb5d3
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>