Focus issues for modals.

[hydra]

In PR #238 there is an example called modal-split.rs which shows how to create two independent modals. In testing keyboard support for this new example the following issues are observed:

1. if you open both modals, and then press tab it's possible to focus the buttons behind the modals by pressing tab, as per this video.

Screen.Recording.2025-01-13.112523.mp4

• the elements behind the modals should not be able to be focused.

2. pressing 'enter' should activate the focused modal's default button.
   If you open both the left and right modals, then use the keyboard to move the focus to the left modal, then press 'enter' the model on the right closes. See also Support multiple default widgets #219.

Screen.Recording.2025-01-13.112402.mp4

• pressing enter should activate the 'default' button for the modal with the focus.