Reorganize question mode

No real code has changed here (only a few comments).
Split sx-question-mode into two files.

- sx-question-print holds all the logic for printing the buffer's content.
- The original file is reserved for the mode definition, interactive commands, and functions that create and display the bufer.

This is based on the previous PR. So merge that first.

because special-mode already makes the buffer read-only. so it was redundant.