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/qtwidgets/Platform_qtwidgets.cpp

@@ -22,7 +22,6 @@
 
 #include <memory.h>
 
-static KDDockWidgets::Platform_qtwidgets s_platformQtWidgets;
 
 using namespace KDDockWidgets;