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/dialog.h

@@ -9,7 +9,7 @@
 #define _WX_QT_DIALOG_H_
 
 #include "wx/dialog.h"
-#include <QtWidgets/QDialog>
+class QDialog;
 
 class WXDLLIMPEXP_CORE wxDialog : public wxDialogBase
 {
@@ -35,7 +35,7 @@ public:
     virtual void EndModal(int retCode);
     virtual bool IsModal() const;
 
-    virtual QDialog *GetHandle() const;
+    QDialog *GetDialogHandle() const;
 
 private: