Fixed resetting icons that outlived their owner.

ChatStyle::value() hands its result to the caller, so the icons it makes
are not ours to keep in _ownedIcons: the lifetime overload's icons die
with the caller - a section's corner buttons, for instance, while the
session style lives on - and the by-value overload's live in a local.
A later palette change then reset through freed pointers.

Track only what the lifetime overload creates, and drop it again when
that lifetime ends.