Include QRect, QSize and QString from wx/qt/private/converter.h

Similarly to the previous commit, it doesn't seem to be worth it to
avoid including these simple and common headers from there and it
simplifies code and will allow making the converter functions inline.

src/qt/converter.cpp

@@ -13,10 +13,7 @@
     #pragma hdrstop
 #endif
 
-#include <QtCore/QRect>
-#include <QtCore/QString>
 #include <QtGui/QFont>
-#include <QtCore/QSize>
 
 #if wxUSE_DATETIME
     #include "wx/datetime.h"