Make it simpler to use mnemonics for wxRadioBox items in XRC

Allow specifying label="1" attribute in wxRadioBox <item> tags to
indicate that the usual translation of "_" to "&" should be done, as for
all the other labels.

This is still not the default behaviour to avoid breaking any existing
XRC files using "_", even though using labels="1" by default would make
more sense.

utils/wxrc/wxrc.cpp

@@ -981,9 +981,14 @@ GetNodeContentsKind(wxXmlNode& node, const wxString& contents)
     }
 
     // This one is special: it is translated in XRC, but its contents is not
-    // escaped.
+    // escaped, except for the special case of wxRadioBox when it can be, if
+    // "label" attribute is supplied.
     if ( node.GetName() == wxT("item") )
-        return Contents_TransOnly;
+    {
+        return node.GetAttribute(wxT("label"), wxT("0")) == wxT("1")
+                ? Contents_Text
+                : Contents_TransOnly;
+    }
 
     return Contents_NotTrans;
 }