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
83e2f8c993