[IMP] new autocomplete widget

Task #5009

This new autocomplete widget (the one used in the search bar) does not
do remote calls automatically, but on demand. In theory, it should lead
to a better user experience, not having the ui blocked every time long
remote calls are done.

It also has the benefits of bringing us one step closer to not
depending on jquery.ui. Bonus point: the code is quite short (< 200 loc
i believe)