Enable High-DPI support for Qt 5.6 and higher

This fixes size hint issues in the event editor on Windows, at
least for me.

Fixes #269

Change-Id: I2a31c9c0aaa4275c17d57ab6e17e84adcb0f1339
Reviewed-on: https://codereview.kdab.com/32523
Tested-by: Continuous Integration <build@kdab.com>
Reviewed-by: Hannah von Reth <hannah.vonreth@kdab.com>

Charm/Charm.cpp

@@ -86,6 +86,9 @@ int main ( int argc, char** argv )
     }
 
     try {
+#if QT_VERSION >= QT_VERSION_CHECK(5,6,0)
+        QGuiApplication::setAttribute( Qt::AA_EnableHighDpiScaling );
+#endif
         QApplication app( argc, argv );
         const std::shared_ptr<ApplicationCore> core( createApplicationCore( startupTask ) );
         QObject::connect( &app, SIGNAL(commitDataRequest(QSessionManager&)), core.get(), SLOT(commitData(QSessionManager&)) );