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/mdi_with_docking/main.cpp

@@ -32,6 +32,8 @@ int main(int argc, char **argv)
     app.setOrganizationName(QStringLiteral("KDAB"));
     app.setApplicationName(QStringLiteral("App supporting both docking and a MDI area"));
 
+    KDDockWidgets::initFrontend(KDDockWidgets::FrontendType::QtWidgets);
+
     QCommandLineParser parser;
     parser.setApplicationDescription("KDDockWidgets MDI mixed with normal docking");
     parser.addHelpOption();