Fix element identifiers not being accepted by tpt __newindex

Becuase *of course* whether it succeds or not depends on whether you've rebuilt menus.

With all seriousness, the real culprit here is GameModel::GetToolFromIdentifier, which looks for tools in the menu section tool lists, plus another list with tools that aren't in any menu section. This is absolutely blaphemous, but I don't feel like refactoring this right now. It also wouldn't be a problem in itself, but allocating an element also doesn't rebuild these lists, only changing the MenuSection property and a few other obscure operations do. This makes allocating elements also rebuild these lists.

src/lua/LuaScriptInterface.cpp

@@ -2827,6 +2827,7 @@ int LuaScriptInterface::elements_allocate(lua_State * l)
 			luacon_ci->custom_can_move[elem][newID] = 0;
 			luacon_ci->custom_can_move[newID][elem] = 0;
 		}
+		luacon_model->BuildMenus();
 		luacon_ci->custom_init_can_move();
 	}