Async support for intentions list

[hedefalk]

To be able to use this I think we need async support. It'd also be super-cool if it could support nested menus with an async hook on each level. I'm thinking

rename
add import ->
   (calls import suggestion async)
   import java.util.Thang
   import scala.Thang 

[steelbrain]

It already supports async callbacks, just return a promise

[hedefalk]

Great!