Implement GetStringSelection() in generic wxBitmapComboBox

Also update the test to avoid assuming that wxBitmapComboBox inherits
from wxComboBox, which now allows it to build (and pass) on all
platforms.

Closes https://github.com/wxWidgets/wxWidgets/pull/2057

include/wx/generic/bmpcbox.h

@@ -84,6 +84,8 @@ public:
 
     virtual ~wxBitmapComboBox();
 
+    virtual wxString GetStringSelection() const wxOVERRIDE;
+
     // Adds item with image to the end of the combo box.
     int Append(const wxString& item, const wxBitmap& bitmap = wxNullBitmap);
     int Append(const wxString& item, const wxBitmap& bitmap, void *clientData);