Review / simplify `FileChooser` based on real-world usage

[amcclain]

Expanding ticket to more general review of this comp and the various apps where we decided not to use it and had to roll our own.

---

The FileChooser component accepts a targetText prop, but always shows its own text when actively hovering over the target. It does not provide any control over the text displayed when a drop is rejected.

https://github.com/xh/hoist-react/blob/9538b51b8e8e8c23755d4ee9b215504a2b80f2a5/desktop/cmp/filechooser/FileChooser.ts#L93

We have a usage in a client app where we are showing the target in a tightly-managed layout and want the ability to customize both.