Keymap: XD75 keymap (#3553)

Keymap: XD75 keymap (#3553)

* cloned colinta as base

* switched to default base. started customising

* minor changes to layout

* added mouse keys

* minor tweaks to layout and mouse speed

* updated readme

* tweaked layout

- added middle fn keys
- moved rgb controls to fn layer middle
- added special keys to fn layer right

* moved scroll buttons as to not interfere with lshift

* turned on backlight

* fixed backlight

* changed user keycodes to differentiate

* changed folder name to lowercase

* updated readme

* stop processing after custom keycode

* edits to suit new standards

* added layer indicator using gp100 and gp103