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/frame.h

@@ -12,8 +12,9 @@
 #define _WX_QT_FRAME_H_
 
 #include "wx/frame.h"
-#include <QtWidgets/QMainWindow>
-#include <QtWidgets/QScrollArea>
+
+class QMainWindow;
+class QScrollArea;
 
 class WXDLLIMPEXP_CORE wxFrame : public wxFrameBase
 {
@@ -45,12 +46,8 @@ public:
     virtual void AddChild( wxWindowBase *child );
     virtual void RemoveChild( wxWindowBase *child );
 
-    virtual QMainWindow *GetHandle() const
-    {
-        return static_cast<QMainWindow*>(m_qtWindow);
-    }
-
-    virtual QAbstractScrollArea *QtGetScrollBarsContainer() const;
+    QMainWindow *GetQMainWindow() const;
+    virtual QScrollArea *QtGetScrollBarsContainer() const;
 
 protected:
     virtual void DoGetClientSize(int *width, int *height) const;