macOS: Adapt new KeyboardEvent on IME event interface

[komi1230]

This is related to #1497 and #1979.
I made is_synthetic field in KeyboardEvent filled with OS's IME status.

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[ArturKovacs]

Could you explain why this change is useful?

[komi1230]

I have misunderstood this is_synthetic field in KeyboardInput means the state of IME.
But the meaning of this field looks more generic event for winit.

I'm closing this PR.