* Converter: add XT USB converter from TMK
* Converter: port TMK code for USB converter to QMK
* Apply requested changes after code review by @drashna
- use "pragma once" instead of include guards
- use LAYOUT instead of KEYMAP
- allow customisation with matrix_init_user() and matrix_scan_user()
- set BOOTLOADER instead of BOOTLOADER_SIZE
- Add XT_ENABLE to OPT_DEFS
* Remove unnecessary lines
