Add support for "cxxcode" typemap

This allows injecting arbitrary code into the generated C++ classes and
can be useful for defining extra constructors or operators, for example.

Doc/Manual/C.html

@@ -496,6 +496,11 @@ area: 7.068583
       <td>Defines how to transform <tt>ctype</tt> value returned by a function to <tt>cxxouttype</tt>
       </td>
   </tr>
+  <tr>
+      <td><tt>cxxcode</tt></td>
+      <td>May contain arbitrary code that will be injected in the declaration of the C++ wrapper class corresponding to the given type. Ignored for non-class types.
+      </td>
+  </tr>
 </table>
 
 <H3>C Typemaps, a Code Generation Walkthrough</H3>

Source/Modules/c.cxx

@@ -528,10 +528,22 @@ public:
 
     Printv(cxx_wrappers_.sect_decls,
       "class ", Getattr(n, "sym:name"), base_classes.get(), " {\n"
-      "public:\n",
+      "public:",
       NIL
     );
 
+    // If we have any extra code, inject it. Note that we need a hack with an artificial extra node to use Swig_typemap_lookup(), as it needs a "type" attribute
+    // which the class node doesn't have.
+    scoped_dohptr dummy(NewHash());
+    Setattr(dummy, "type", Getattr(n, "name"));
+    Setfile(dummy, Getfile(n));
+    Setline(dummy, Getline(n));
+    scoped_dohptr cxxcode(Swig_typemap_lookup("cxxcode", dummy, "", NULL));
+    if (!cxxcode || *Char(cxxcode) != '\n')
+      Append(cxx_wrappers_.sect_decls, "\n");
+    if (cxxcode)
+      Append(cxx_wrappers_.sect_decls, cxxcode);
+
     class_node_ = n;
     dtor_wname_ = NULL;
     has_copy_ctor_ = false;