added a new operator for ItemData to cast type to bool.

Current version of ItemData has '==' operator. So ItemBool instances
can be comparared like,

void foo(ItemBool *item)
{
  ItemBool *itemTrue(true);
  if (*item == *itemTrue)
    ....

However, this patch makes that situation more simple as the following.

void foo(ItemBool *item)
{
  if (*item)
    ...