Add `asyncControl` to `EditableText`

[felixgabler]

Fixes #4678

Checklist

• [ ] Includes tests
• [ ] Update documentation

Changes proposed in this pull request:

Similarly to InputGroup, EditableText should support the asyncControl prop.

Reviewers should focus on:

Currently, I reuse AsyncControllableInput but also had to make that class support the multiline scenario. I wonder whether it is better to copy this class to a AsyncControllableTextArea and figure out another way of sharing code instead of having one class. What do you think?

After that decision is made, I will gladly write more tests if required. Any pointers here would also be welcome.

Screenshot

[palantirtech]

Thanks for your interest in palantir/blueprint, @felixgabler! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.