LISTBOX_INVALID value required

There is this:
```c++
enum
{
    LISTBOX_TEXT = 1,
    LISTBOX_IMAGE = 2,
};
```
I think we should also add
```c++
LISTBOX_INVALID = 0
```
Reason being, 0 is reserved for list box tests when it is memset to 0. If anyone tried to add a new list box type at 0, it would not work correctly. 0 is treated as invalid type.