Fix forward declaration. Remove not necessary includes

Change-Id: If2dc2c52fa8dbabd9aeca03ba96e01c1c6bce37d
Reviewed-on: https://codereview.kdab.com/68186
Reviewed-by: Sérgio Martins <sergio@kdab.com>

src/MainWindow.cpp

@@ -25,7 +25,6 @@
 #include "DockRegistry_p.h"
 #include "multisplitter/MultiSplitter_p.h"
 
-#include <QPainter>
 #include <QApplication>
 
 using namespace KDDockWidgets;