LISTBOX_INVALID value required

[xezon]

There is this:

enum
{
    LISTBOX_TEXT = 1,
    LISTBOX_IMAGE = 2,
};

I think we should also add

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.