Chris Bracken c3a0385fb5 Use common desktop TextInputModel on macOS (flutter/engine#24533)
This replaces the custom Obj-C TextInputModel implementation used on
macOS with the common C++ implementation used on Linux and Windows. Note
that as a side-effect, this change enables *some* direct IME input for
CJK input but full input will land in follow-up patches that land:

1. Add handling for TextInput.setMarkedTextRect message
2. Add handling for TextInput.setEditableSizeAndTransform message
3. Implement firstRectForCharacterRange:actualRange: using the above.
4. Add NSTextInputContext handling.
2021-02-25 00:04:54 -08:00
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