Move wxComboBox to wxBitmapComboBox in hopes it will work better.

(Not sure if it solves the MSW issues yet, but at least it highlights
the items in the dropdown on rollover on OSX.)

Also added some text to the dropdown menu for intersheet references in
the PDF.

common/plotters/PDF_plotter.cpp

@@ -978,7 +978,7 @@ bool PDF_PLOTTER::EndPlot()
         for( const std::pair<wxString, int>& page : pages )
         {
             js += wxString::Format( wxT( "{ cName: '%s', cReturn: '%d' }, " ),
-                                    page.first,
+                                    wxString::Format( _( "Show Page %s" ), page.first ),
                                     page.second );
         }