OpenHead for FlyingBear Tornado2 (FB T2)

This is my design of FlyingBear Tornado2 (T2) head. It's called OpenHead not only because it is fully open-source, but also because it incorporates open structure without any enclosures (unlike the stock T2 head, which is enclosed cube), providing you easy access for tuning and maintenance. The main benefits by comparison with the stock T2 head are:
- very lightweight, allowing you to increase print quality among with really high acceleration values;
- very easy to assemble, only 4 main parts to install (v6Head, v6Holder, Fan_holder, v6Duct_v2);
- very easy to clean, service and replace broken parts or the entire assembly;
- better cable chain and auxiliary board holder, replaces ugly and heavy stock holder; it has two thickness options: 3.5mm (suitable for most of applications) and 5mm (for really heavy work);
- better print quality due to use of higher quality hotend and fan;
- easy to print even with stock T2 head;
- fan is adjustable by Z axis;
- easier to see currently pri