Improve calculating wxButton best size under wxMSW

We can use BCM_GETIDEALSIZE message to get the size of the button that
best fits its text and image (if present).
It gives better results for text wxButton with wxBU_EXACTFIT style.

include/wx/msw/anybutton.h

@@ -59,6 +59,7 @@ protected:
     // Should only be called if we do have a button, i.e. if m_imageData is
     // non-NULL.
     void AdjustForBitmapSize(wxSize& size) const;
+    void AdjustForBitmapMargins(wxSize& size) const;
 
     class wxButtonImageData *m_imageData;