Ensure focus trap, Tabs and Dialog play well together (#1231)

* add internal FocusSentinel component

This component will allow you to catch the focus and forward it to a new
element. The catch is that it will retry to do that because sometimes
components won't be available yet.

E.g.: We want to focus the first Tab component if it is rendered inside
the Dialog. However, a Tab will register itself in the next tick,
triggering a re-render and only then will it be `selected`. This is a
bit too late for the FocusTrap component.

The FocusSentinel should fix this by catching the focus, and forwarding
it to the correct component. Once that is done, it will remove itself
from the DOM tree so that you can't ever focus that element anymore.
This should fix potential `<tab>` and `<shift+tab>` behaviour.

* find the selectedIndex asap

* use the FocusSentinel and forward it to the correct Tab

* add example Tab in Dialog example

* suppress console warnings

Because we are firing `setState` calls within the component, React is
yelling at us for not using `act(() => { ... })`. Welp, not going to add
those calls inside the component just for tests...

* update changelog

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Remove `focus()` from Listbox Option ([#1218](https://github.com/tailwindlabs/headlessui/pull/1218))
 - Improve some internal code ([#1221](https://github.com/tailwindlabs/headlessui/pull/1221))
 - Use `ownerDocument` instead of `document` ([#1158](https://github.com/tailwindlabs/headlessui/pull/1158))
+- Ensure focus trap, Tabs and Dialog play well together ([#1231](https://github.com/tailwindlabs/headlessui/pull/1231))
 
 ### Added
 
@@ -52,6 +53,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Don’t drop initial character when searching in Combobox ([#1223](https://github.com/tailwindlabs/headlessui/pull/1223))
 - Use `ownerDocument` instead of `document` ([#1158](https://github.com/tailwindlabs/headlessui/pull/1158))
 - Re-expose `el` ([#1230](https://github.com/tailwindlabs/headlessui/pull/1230))
+- Ensure focus trap, Tabs and Dialog play well together ([#1231](https://github.com/tailwindlabs/headlessui/pull/1231))
 
 ### Added