Deprecate QApplication::setGraphicsSystem

Deprecate QApplication::setGraphicsSystem

The QApplication::setGraphicsSystem() call is entirely vestigial after the
QPA rearchitecting of Qt. Deprecate it to give people a heads up that their
original intent no longer has any impact on their application.

Change-Id: I21838431db1057f7f7858b4a434ae0eebd2317de
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>