callback constructor for yewtil LinkFuture trait

[kellpossible]

it would be nice to be able to have a callback constructor equivalent to ComponentLink::callback() which executes callbacks in a future with spawn_local. I have a proof of concept at kellpossible/coster@1f9006e#diff-e8709839a6bad6639c9bbaa6aea8db01R96 and it seems to work well for my purpose. Perhaps the new method could be called LinkFuture::callback_future().

[mc1098]

#1842 fixed this :)
Scope (ComponentLink) now has callback_future