giving a try at simplifying auto_complete

If I understand correctly the `skipSearch` boolean is a way for `AutoCompleteElement` of protecting itself from the fact that it has bound any change to `search` to `_performSearch`. So it could be avoided if there was a way of telling "update `search` but please don't notify listeners`. This is what I tried to achieve with `_setSearch` in this CL. It works but I'm not sure it's clearer :)

So consider this pull request as a discussion rather than an actual pull request (unless you find this solution more elegant).