Implemented dynamic deletion and addition of items to menu

The code works for the current menu data structure and can be applied only to
the menubar and its sub-menus (it was tested only for them). So, the value passed
to Menu parameter will be one of the top menubar options (file,edit,tool,...).
Removing => update_menu(Menu, Item, delete, _)
Adding   => update_menu(Menu, Item, {append, Pos0, Cmd0}, Help)

A sample of use can be found in YafaRay plugin in which it remove the render
option in case a valid path to the application was not set:
Removing => wings_menu:update_menu(file, {render, ?TAG}, delete);
Adding   => wings_menu:update_menu(file, {render, ?TAG}, {append, -1, Label})

The value -1 for the append operation means the item will be appended to the end
of the menu/sub-menu; Otherwise, the item will be inserted in the specificated
position or to the end of the menu if this value is greater than current amount
of menu items.

NOTE: Added logic that allow the Yafaray - after the engine path be provided -
get its render option available under File->Render option whithout need to
restart Wings3D. Thanks oort for ask about it.

plugins_src/import_export/wpc_yafaray.erl

@@ -2147,7 +2147,16 @@ pref_dialog(St) ->
 
 pref_result(Attr, St) ->
     set_user_prefs(Attr),
+    OldVal = get_var(renderer),
     init_pref(),
+    case get_var(renderer) of
+        OldVal -> ok;
+        false ->
+            wings_menu:update_menu(file, {render, ?TAG}, delete);
+        _ ->
+            [{Label, _}] = menu_entry(render),
+            wings_menu:update_menu(file, {render, ?TAG}, {append, -1, Label})
+    end,
     St.
 
 export_dialog(Op, Title) ->

src/wings_menu.erl

@@ -124,6 +124,11 @@ build_command(Name, Names, true) ->
 build_command(Name, Names, false) ->
     build_command(Name, Names).
 
+build_names(Term, Acc) when not is_tuple(Term) -> Acc;
+build_names({_,Term2}, Acc) when is_boolean(Term2) -> Acc;
+build_names({Term1,Term2}, Acc) ->
+    build_names(Term2, [Term1]++Acc).
+
 have_option_box(Ps) ->
     proplists:is_defined(option, Ps).
 
@@ -589,6 +594,49 @@ update_menu(file, Item = {recent_file, _}, delete, _) ->
     true = wxMenu:delete(File, Id),
     ets:delete(wings_menus, Id),
     ok;
+update_menu(Menu, Item, delete, _) ->
+    case menu_item_id(Menu, Item) of
+	false -> ok;
+	Id ->
+	    case ets:lookup(wings_menus, Id) of
+		[#menu{object=MenuItem}] ->
+		    ParentMenu = wxMenuItem:getMenu(MenuItem),
+		    true = wxMenu:delete(ParentMenu, Id),
+		    ets:delete(wings_menus, Id),
+		    ok;
+		_ ->
+		    ok
+	    end
+    end;
+update_menu(Menu, Item, {append, Pos0, Cmd0}, Help) ->
+    case menu_item_id(Menu, Item) of
+	false ->
+	    AddItem =
+		fun(SubMenu, Name) ->
+		    Pos =
+			if Pos0 >= 0 -> min(wxMenu:getMenuItemCount(SubMenu), Pos0);
+			true -> wxMenu:getMenuItemCount(SubMenu)
+			end,
+		    MO = wxMenu:insert(SubMenu, Pos, -1, [{text, Cmd0}]),
+		    Id = wxMenuItem:getId(MO),
+		    ME=#menu{name=Name, object=MO,
+			     wxid=Id, type=?wxITEM_NORMAL},
+		    true = ets:insert(wings_menus, ME),
+		    Cmd = setup_hotkey(MO, Cmd0),
+		    wxMenuItem:setText(MO, Cmd),
+		    is_list(Help) andalso wxMenuItem:setHelp(MO, Help)
+		end,
+
+	    Names = build_names(Item, [Menu]),
+	    case ets:match_object(wings_menus, #menu{name=Names, _ = '_'}) of
+		[#menu{object=SubMenu}] ->
+		    AddItem(SubMenu, build_command(Item, [Menu]));
+		_ ->
+		    io:format("update_menu: Item rejected (~p|~p)\n",[Menu,Item])
+	    end;
+	_ ->
+	    ok
+    end;
 update_menu(Menu, Item, Cmd0, Help) ->
     Id = menu_item_id(Menu, Item),
     MI = case ets:lookup(wings_menus, Id) of
@@ -810,7 +858,7 @@ menu_item_desc(Desc, HotKey) ->
 	_ -> Desc ++ "\t" ++ HotKey
     end.
 
-%% We want to use the prefdefined id where they exist (mac) needs for it's
+%% We want to use the predefined id where they exist (mac) needs for it's
 %% specialized menus but we want our shortcuts hmm.
 %% We also get little predefined icons for OS's that have that.
 predefined_item(Menu, Item, DefId) ->