TextInput: refresh the adornment API

[bpierre]

See discussion: https://github.com/aragon/aragon-ui/pull/612#discussion_r342338026

Might also fix https://github.com/aragon/aragon-ui/issues/534 at the same time.

[stvtortora]

Hi, I am working on this enhancement. Does this need to be backward-compatible? If not, will all uses of the TextInput component in your other repositories need to be updated as well?

[sohkai]

@stvtortora Ideally we can still make the API backwards compatible and warn developers that they should use the new one.

You'll see we have a few utilities to help log and warn in these cases :).

Don't worry about updating everywhere else—that will be done as we upgrade their aragonUI versions!