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/notebook.h

@@ -8,7 +8,7 @@
 #ifndef _WX_QT_NOTEBOOK_H_
 #define _WX_QT_NOTEBOOK_H_
 
-#include <QtWidgets/QTabWidget>
+class QTabWidget;
 
 class WXDLLIMPEXP_CORE wxNotebook : public wxNotebookBase
 {
@@ -45,7 +45,7 @@ public:
     int SetSelection(size_t nPage) { return DoSetSelection(nPage, SetSelection_SendEvent); }
     int ChangeSelection(size_t nPage) { return DoSetSelection(nPage); }
 
-    virtual QTabWidget *GetHandle() const;
+    virtual QWidget *GetHandle() const;
 
 protected:
     virtual wxWindow *DoRemovePage(size_t page);