Blender Git Loki

macOS: support Japanese input for text buttons

Blender did not support to input East Asian characters (Chinese, Japanese,
Korean) on macOS. This patch adds support for Japanese input, by implementing
the appropriate processing for the NSTextInputClient protocol.

Technical notes:
* The conversion candidate window is drawn by the input method program calling
`firstRectForCharacterRange`.
* The string before confirmation (called `composite` in blender) is handled in
the `setMarkedText` method called by the input method program.
* The string after confirmation (called `result` in the blender) is processed
in the `insertText` method called by the input method program.

Ref T51283

Differential Revision: https://developer.blender.org/D11695