Consider adding "ReplaceLastCharacter" event for MacOS accent input

[kettle11]

While investigating https://github.com/kettle11/kapp/issues/64 I came across a MacOS only feature that won't work correctly with kapp's text input events.

When holding the 'a' key the following things occur:

• CharacterReceived { character: 'a' } is sent
• A candidate window appears with different accents over 'a'
• If an accent is selected then CharacterReceived { character: 'à' } is sent.

With kapp's current model there is no way to indicate that the last character should be replaced.

@lunabunn investigated and concluded that perhaps a Mac only "ReplaceLastCharacter" or similar event could be emitted. For now there's not an immediate need for this event, but later it may be needed to make MacOS text input more correct.