Associate an opaque data pointer with the item at index.
The pointer is stored inside the native control, where the D garbage
collector cannot see it. To keep GC-allocated data from being collected
out from under the control, Deft also retains a reference internally for the
control's lifetime; the retained references are released by clear().
Associate an opaque data pointer with the item at index.
The pointer is stored inside the native control, where the D garbage collector cannot see it. To keep GC-allocated data from being collected out from under the control, Deft also retains a reference internally for the control's lifetime; the retained references are released by clear().