Fix wxBitmapComboBox size in high DPI

Convert hard-coded sizes to DPI depended size, remove unused size.
Implement MSWUpdateFontOnDPIChange() and update the control elements.

include/wx/msw/bmpcbox.h

@@ -104,6 +104,7 @@ protected:
     WXDWORD MSWGetStyle(long style, WXDWORD *exstyle) const wxOVERRIDE;
     virtual bool MSWOnDraw(WXDRAWITEMSTRUCT *item) wxOVERRIDE;
     virtual bool MSWOnMeasure(WXMEASUREITEMSTRUCT *item) wxOVERRIDE;
+    virtual void MSWUpdateFontOnDPIChange(const wxSize& newDPI) wxOVERRIDE;
 
     // Event handlers
     void OnSize(wxSizeEvent& event);