Try fix Korean IME on iOS

[kerams]

What does the pull request do?

Tries to fix #16956 on iOS.

What is the current behavior?

What is the updated/expected behavior with this PR?

See #16956

How was the solution implemented (if it's not obvious)?

With ㅅ in a text box, when the user types ㅓ, iOS invokes IUIKeyInput.DeleteBackward to remove ㅅ, and then IUIKeyInput.InsertText with 서. Further typing ㄴ should result in another delete followed by the insertion of 선.

However, the second IUIKeyInput.DeleteBackward does not currently happen and ㄴ is inserted instead, producing 서ㄴ, probably due to how the previous delete was handled internally (resulting in the wrong return value from IUITextInput.TextInRange). With my change it is possible to type 3- and 4-character Korean syllabic blocks, but on only the first syllable. Additionally, manual deletion does not remove the entire syllable but only its last part (선 becomes 서 when deleting), which is the norm for Korean.

Sadly, further syllables continue to be broken, and manual deletes without calling keydown only select the final character. I'd appreciate any tips, especially describing why and how the double key up/down backspace invocation is required.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

[kerams]

Does Avalonia do any internal postprocessing of text entered in a TextBox? When I set the implementation of IUIKeyInput.DeleteBackward to be empty, I'd expect to see ㅅ서선 when typing ㅅ, ㅓ, ㄴ, because IUIKeyInput.InsertText appends text, yet the text box correctly contains just 선 despite disregarding the issued deletes.

I don't know how to explain this without the said postprocessing, and not sure I can fix the problem without understanding how this behavior is possible.