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

src/qtquick/Platform_qtquick.cpp

@@ -22,8 +22,6 @@
 #include <QTimer>
 #include <QWindow>
 
-static KDDockWidgets::Platform_qtquick s_platformQtQuick;
-
 using namespace KDDockWidgets;