Readonly Textinput beeps on focus loss with keyboard on GTK

### Describe the bug

Under GTK, If a readonly text input loses focus as a result of keyboard input (i.e., pressing tab), a beep is emitted.

### Steps to reproduce

1. Run Tutorial 1
2. Type Tab twice

A beep will be emitted when the Celsius field loses focus.

### Expected behavior

No beep should be emitted.

### Screenshots

_No response_

### Environment

- Operating System: Ubuntu 22.04
- Python version: All
- Software versions:
- Toga: 0.4.2+

### Logs

```

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### Additional context

No beep is emitted on *loss* of focus.

It's arguable whether the readonly widget should be able to accept focus at all - that's the behavior on macOS. If this is acceptable, calling `native.set_can_focus()` as part of `set_readonly()` might be a fix for this.

There is likely similar issues on any other widget with a clear input focus and a readonly option.

Reproduce the issue by running Tutorial 1 on Ubuntu 22.04 and pressing Tab twice, then inspect the readonly text input's native focus handling. Consider the mentioned set_can_focus() path and check whether other readonly widgets show the same behavior. Done means keyboard focus changes no longer emit a beep.

Written by the indexing model from the issue text.