gtk2: use long instead of int

Because XChangeProperty(3) says:

> If the specified format is 8, the property data must be a char array.
>
> If the specified format is 16, the property data must be a short array.

> If the specified format is 32, the property data must be a long array.

In this case (format is 32 case), we should use a long array instead of
an int array.

GitHub: #256

Patch by mtasaka. Thanks!!!