Improve build and widget storage

There are no longer any qt headers included in wx/qt headers.
Applications do not need to link with qt librarys anymore, only wxqt libraries.
wxWindow and derived widgets only contain one pointer to their qtwidget, no longer
  carrying both base and derived pointers in parallel as was before.

include/wx/qt/listctrl.h

@@ -9,7 +9,9 @@
 #define _WX_QT_LISTCTRL_H_
 
 #include "wx/textctrl.h"
-#include <QtWidgets/QTreeWidget>
+
+class QTreeWidget;
+class QTreeWidgetItem;
 
 class WXDLLIMPEXP_FWD_CORE wxImageList;
 
@@ -284,7 +286,7 @@ public:
         return OnGetItemAttr(item);
     }
 
-    virtual QTreeWidget *GetHandle() const;
+    virtual QWidget *GetHandle() const;
 
 protected:
     void Init();