Blender Git Loki

Store search item in search-button data, don't reuse general void pointer

Rather than re-using the `uiBut.func_arg2` pointer to reference the active
search item, have an own pointer for that in `uiButSearch`.

Unfortunately, we still need to pass the reference as `arg2` to the
`uiBut.func` callback. But we don't need to keep `uiBut.func_arg2` set the
entire time for that.