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.

docs/doxygen/overviews/xrc_format.h

@@ -1595,6 +1595,11 @@ can also have some optional XML @em attributes (not properties!):
      Is the button enabled (default: 1)?}
 @row3col{hidden, @ref overview_xrcformat_type_bool,
      Is the button hidden initially (default: 0)?}
+@row3col{label, @ref overview_xrcformat_type_bool,
+     Should this item text be interpreted as a label, i.e. escaping underscores
+     in it as done for the label properties of other controls? This attribute
+     exists since wxWidgets 3.1.1 and was always treated as having the value of
+     0, which still remains its default, until then.}
 @endTable
 
 Example: