Add KDDockWidgets::initPlatform()

Instead of using the static variable, which is initialized before
having QApplication.

Will allow us to remove the QTimer from Platform init

examples/dockwidgets/main.cpp

@@ -33,10 +33,11 @@ int main(int argc, char **argv)
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
 #endif
     QApplication app(argc, argv);
-
     app.setOrganizationName(QStringLiteral("KDAB"));
     app.setApplicationName(QStringLiteral("Test app"));
 
+    KDDockWidgets::initFrontend(KDDockWidgets::FrontendType::QtWidgets);
+
     QCommandLineParser parser;
     parser.setApplicationDescription("KDDockWidgets example application");
     parser.addHelpOption();