Disallow copy and cut when `obscureText` is set on `TextField` (#96233)

Before this change, it was possible to select and copy obscured text from a text field.

This PR changes things so that:

- Obscured text fields don't allow copy or cut.
- If a field is both obscured and read-only, then selection is disabled as well (if you can't modify it, and can't copy it, there's no point in selecting it).