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/CMakeLists.txt

@@ -22,6 +22,7 @@ add_definitions(-DQT_NO_SIGNALS_SLOTS_KEYWORDS
 )
 
 set(DOCKSLIBS_SRCS
+    KDDockWidgets.cpp
     Config.cpp
     Config.h
     Qt5Qt6Compat_p.h