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:

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
             }*
         }?

src/xrc/xh_radbx.cpp

@@ -107,7 +107,13 @@ wxObject *wxRadioBoxXmlHandler::DoCreateResource()
         // we handle handle <item>Label</item> constructs here, and the item
         // tag can have tooltip, helptext, enabled and hidden attributes
 
-        m_labels.push_back(GetNodeText(m_node, wxXRC_TEXT_NO_ESCAPE));
+        // For compatibility, labels are not escaped in XRC by default and
+        // label="1" attribute needs to be explicitly specified to handle them
+        // consistently with the other labels.
+        m_labels.push_back(GetNodeText(m_node,
+                                       GetBoolAttr("label", 0)
+                                        ? 0
+                                        : wxXRC_TEXT_NO_ESCAPE));
 #if wxUSE_TOOLTIPS
         m_tooltips.push_back(GetNodeText(GetParamNode(wxT("tooltip")),
                                          wxXRC_TEXT_NO_ESCAPE));

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;
 }