Fixed acting on the wrong sticker in a mixed set.

Hit testing yields an index into _elements, which skips the premium
stickers an account that cannot get premium must not see, while preview,
sending, the context menu, deletion and reordering all looked that index
up in _pack - which keeps them. With one skipped sticker before a visible
one, clicking a sticker sent a different one.

Everything driven by a visible position now reads _elements, and the
reorder translates both positions through the documents so the full pack
and the server still get the position in the whole set.