Refactor QSGVisualDataModel.

Split the wrapper for Package models out into its own visual model
class, add a QSGVisualAdaptorModel class which encapsulates the
creation and maintenance of delegate context data, and move
QSGVisualDataModel to its own source files.

Task-number: QTBUG-20107
Change-Id: Icdea2756a119c327ec2c748be893daf622bc1356
Reviewed-on: http://codereview.qt-project.org/4033


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>