Model::clear() cloned the entire order VecDeque to iterate while
remove() mutated it, producing an O(n) allocation proportional to the
number of items — needless on a clear() which is going to drop all of
them anyway.
Replace the clone with std::mem::take(&mut self.order): we iterate the
taken VecDeque (transferring ownership), and the inner self.order.remove(index)
in each remove() call now finds position()==None and no-ops, since
self.order has been swapped with an empty default.
Same semantics, zero allocation. Noticeable on large nav/table models
(>100 items) and on apps that reset state frequently (settings pages,
file lists, context menus).
