* Remove unnecessary IS_COMMAND definition in clueboard/66/rev4
* Replace keyboard_report->mods with get_mods() in IS_COMMAND definitions
* Update docs to use get_mods() instead of keyboard_report->mods
* Replace keyboard_report->mods with get_mods() in tmk_core/protocol/usb_hid/test
For some reason, this occurrence used KB_LSHIFT and KB_RSHIFT, which don't exist
* Replace keyboard_report->mods with get_mods() in Massdrop keyboards (as per @patrickmt)
* 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
