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>

src/widgets/kernel/qapplication.h

@@ -111,8 +111,9 @@ public:
     enum ColorSpec { NormalColor=0, CustomColor=1, ManyColor=2 };
     static int colorSpec();
     static void setColorSpec(int);
-    // ### Qt4 compatibility, remove?
-    static inline void setGraphicsSystem(const QString &) {}
+#if QT_DEPRECATED_SINCE(5, 0)
+    QT_DEPRECATED static inline void setGraphicsSystem(const QString &) {}
+#endif
 
     using QGuiApplication::palette;
     static QPalette palette(const QWidget *);