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.

misc/schema/xrc_schema.rnc

@@ -1225,6 +1225,7 @@ wxRadioBox =
                 attribute helptext { t_string }?,
                 attribute enabled  { t_bool }?,
                 attribute hidden   { t_bool }?,
+                attribute label    { t_bool }?,
                 t_text
             }*
         }?