## Description
This PR fixes a focus traversal issue for read-only TextField on macOS.
# Implementation details
On macOS, some editing capabilities are handled differently compared to
other platforms.
Since https://github.com/flutter/flutter/pull/105407, the macOS engine
send editing selectors to the framework.
To do so a text input connection should be opened.
Before this PR there was no text input connection for a read-only
EditableText which means several shortcut were not handled, especially
tab traversal (but also selection shortcuts using arrows).
After this PR an input connection is always created on macOS even if an
EditableText is read-only.
## Related Issue
Fixes [Read-only TextField prevents focus from
changing](https://github.com/flutter/flutter/issues/161482)
## Tests
Adds 1 test.
b85c0fc2fe